chore: Fix installing from GitHub URL

[jackfranklin]

The change to the install script to require TypeScript works fine when
installing from npm (because on npm the lib directory with the
compiled code already exists) but doesn't if you install from a GitHub
URL. By default it seems npm uses the files list when you install from
GitHub which means it's missing a bunch of files that we need to
compile.

Additionally by default when installing from a GitHub URL npm doesn't
install the dependencies which is an issue for us when we need to
compile TypeScript.

The fix is to create a prepare script that runs TypeScript if
required. From the npm docs 1:

prepare: Run both BEFORE the package is packed and published, on
local npm install without any arguments, and when installing git
dependencies

And from the npm docs on install 2, it confirms that if a package has
a prepare script it is run when installing from GitHub:

As with regular git dependencies, dependencies and devDependencies
will be installed if the package has a prepare script, before the
package is done installing.

Despite having the prepare script we still need the TypeScript check
in install.js to satisfy the 3rd scenario below where we need to force
a compile:

• If I'm a user installing puppeteer@X from npm, the module is
  published with the lib/ directory of compiled code, so I'm set.
• If I'm a user installing Puppeteer from GitHub, the prepare script
  will run TypeScript for me so I'm set.
• If I'm a developer working on Puppeteer, the prepare script also
  runs but after npm install which means install.js fails as it
  requires ./lib/helper.js. So in install.js we call
  compileTypeScriptIfRequired to catch this case.

Fixes #5660.

[jackfranklin]

Installing from this branch works for me:

npm i --save-dev https://github.com/GoogleChrome/puppeteer#fix-install-from-gh

> puppeteer@3.0.0-post install /Users/jacktfranklin/src/temp/test-pup/node_modules/puppeteer
> node install.js

Downloading Chromium r737027 - 118.4 Mb [====================] 100% 0.0s
Chromium (737027) downloaded to /Users/jacktfranklin/src/temp/test-pup/node_modules/puppeteer/.local-chromium/mac-737027
npm WARN test-pup@1.0.0 No description
npm WARN test-pup@1.0.0 No repository field.

+ puppeteer@3.0.0-post
added 49 packages from 77 contributors and audited 80 packages in 58.06s