feat: Set which browser to launch via PUPPETEER_PRODUCT

This change introduces a PUPPETEER_PRODUCT environment
variable as a first step toward using Puppeteer with
many different browsers. Setting PUPPETEER_PRODUCT=firefox, for
example, enables Firefox-specific Launcher settings.

The state is also exposed as `puppeteer.product` in the API
to support adding other product-specific behaviour as needed.

The bulk of the change is a refactoring in Launcher
to decouple generic browser start-up from product-specific
configuration.

docs/api.md

@@ -31,6 +31,7 @@
   * [puppeteer.errors](#puppeteererrors)
   * [puppeteer.executablePath()](#puppeteerexecutablepath)
   * [puppeteer.launch([options])](#puppeteerlaunchoptions)
+  * [puppeteer.product](#puppeteerproduct)
 - [class: BrowserFetcher](#class-browserfetcher)
   * [browserFetcher.canDownload(revision)](#browserfetchercandownloadrevision)
   * [browserFetcher.download(revision[, progressCallback])](#browserfetcherdownloadrevision-progresscallback)
@@ -388,6 +389,7 @@ If Puppeteer doesn't find them in the environment during the installation step,
 - `PUPPETEER_DOWNLOAD_HOST` - overwrite URL prefix that is used to download Chromium. Note: this includes protocol and might even include path prefix. Defaults to `https://storage.googleapis.com`.
 - `PUPPETEER_CHROMIUM_REVISION` - specify a certain version of Chromium you'd like Puppeteer to use. See [puppeteer.launch([options])](#puppeteerlaunchoptions) on how executable path is inferred. **BEWARE**: Puppeteer is only [guaranteed to work](https://github.com/GoogleChrome/puppeteer/#q-why-doesnt-puppeteer-vxxx-work-with-chromium-vyyy) with the bundled Chromium, use at your own risk.
 - `PUPPETEER_EXECUTABLE_PATH` - specify an executable path to be used in `puppeteer.launch`. See [puppeteer.launch([options])](#puppeteerlaunchoptions) on how the executable path is inferred. **BEWARE**: Puppeteer is only [guaranteed to work](https://github.com/GoogleChrome/puppeteer/#q-why-doesnt-puppeteer-vxxx-work-with-chromium-vyyy) with the bundled Chromium, use at your own risk.
+- `PUPPETEER_PRODUCT` - specify which browser you'd like Puppeteer to use. Must be one of `chrome` or `firefox`. This is exposed in [`puppeteer.product`](#puppeteerproduct)
 
 > **NOTE** PUPPETEER_* env variables are not accounted for in the [`puppeteer-core`](https://www.npmjs.com/package/puppeteer-core) package.
 
@@ -527,7 +529,7 @@ try {
 - `options` <[Object]>  Set of configurable options to set on the browser. Can have the following fields:
   - `ignoreHTTPSErrors` <[boolean]> Whether to ignore HTTPS errors during navigation. Defaults to `false`.
   - `headless` <[boolean]> Whether to run browser in [headless mode](https://developers.google.com/web/updates/2017/04/headless-chrome). Defaults to `true` unless the `devtools` option is `true`.
-  - `executablePath` <[string]> Path to a Chromium or Chrome executable to run instead of the bundled Chromium. If `executablePath` is a relative path, then it is resolved relative to [current working directory](https://nodejs.org/api/process.html#process_process_cwd). **BEWARE**: Puppeteer is only [guaranteed to work](https://github.com/GoogleChrome/puppeteer/#q-why-doesnt-puppeteer-vxxx-work-with-chromium-vyyy) with the bundled Chromium, use at your own risk.
+  - `executablePath` <[string]> Path to a browser executable to run instead of the bundled Chromium. If `executablePath` is a relative path, then it is resolved relative to [current working directory](https://nodejs.org/api/process.html#process_process_cwd). **BEWARE**: Puppeteer is only [guaranteed to work](https://github.com/GoogleChrome/puppeteer/#q-why-doesnt-puppeteer-vxxx-work-with-chromium-vyyy) with the bundled Chromium, use at your own risk.
   - `slowMo` <[number]> Slows down Puppeteer operations by the specified amount of milliseconds. Useful so that you can see what is going on.
   - `defaultViewport` <?[Object]> Sets a consistent viewport for each page. Defaults to an 800x600 viewport. `null` disables the default viewport.
     - `width` <[number]> page width in pixels.
@@ -536,7 +538,7 @@ try {
     - `isMobile` <[boolean]> Whether the `meta viewport` tag is taken into account. Defaults to `false`.
     - `hasTouch`<[boolean]> Specifies if viewport supports touch events. Defaults to `false`
     - `isLandscape` <[boolean]> Specifies if viewport is in landscape mode. Defaults to `false`.
-  - `args` <[Array]<[string]>> Additional arguments to pass to the browser instance. The list of Chromium flags can be found [here](http://peter.sh/experiments/chromium-command-line-switches/).
+  - `args` <[Array]<[string]>> Additional arguments to pass to the browser instance. The list of Chromium flags can be found [here](http://peter.sh/experiments/chromium-command-line-switches/), and here is the list of [Firefox flags](https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options).
   - `ignoreDefaultArgs` <[boolean]|[Array]<[string]>> If `true`, then do not use [`puppeteer.defaultArgs()`](#puppeteerdefaultargsoptions). If an array is given, then filter out the given default arguments. Dangerous option; use with care. Defaults to `false`.
   - `handleSIGINT` <[boolean]> Close the browser process on Ctrl-C. Defaults to `true`.
   - `handleSIGTERM` <[boolean]> Close the browser process on SIGTERM. Defaults to `true`.
@@ -547,6 +549,7 @@ try {
   - `env` <[Object]> Specify environment variables that will be visible to the browser. Defaults to `process.env`.
   - `devtools` <[boolean]> Whether to auto-open a DevTools panel for each tab. If this option is `true`, the `headless` option will be set `false`.
   - `pipe` <[boolean]> Connects to the browser over a pipe instead of a WebSocket. Defaults to `false`.
+  - `extraPrefs` <[Object]> Additional [preferences](https://developer.mozilla.org/en-US/docs/Mozilla/Preferences/Preference_reference) that can be passed to Firefox (see `PUPPETEER_PRODUCT`)
 - returns: <[Promise]<[Browser]>> Promise which resolves to browser instance.
 
 
@@ -565,6 +568,12 @@ const browser = await puppeteer.launch({
 >
 > See [`this article`](https://www.howtogeek.com/202825/what%E2%80%99s-the-difference-between-chromium-and-chrome/) for a description of the differences between Chromium and Chrome. [`This article`](https://chromium.googlesource.com/chromium/src/+/lkgr/docs/chromium_browser_vs_google_chrome.md) describes some differences for Linux users.
 
+#### puppeteer.product
+- returns: <[string]> returns the name of the browser that is under automation ("chrome" or "firefox")
+
+The product is set by the `PUPPETEER_PRODUCT` environment variable and defaults to `chrome`. Firefox support is experimental.
+
+
 ### class: BrowserFetcher
 
 BrowserFetcher can download and manage different versions of Chromium.