New: Add public APIs for config-arrays and file-infos

designs/2019-public-config-array-api/README.md

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+- Start Date: 2019-12-09
+- RFC PR: <!-- leave this empty, to be filled in later -->
+- Authors: @manuth
+
+# Public `ConfigArray` API
+
+## Summary
+
+Allow 3rd-party developers to determine which configuration-files are considered by `eslint`.
+
+## Motivation
+
+In some cases people might want to automatically trigger a new linting process as soon as the configuration of eslint has changed.
+This is a key feature for language servers which provide warnings and fixes to IDEs.
+
+Therefore the language-server needs to know which config-files are considered by `eslint`.
+
+## Detailed Design
+
+A method called `getConfigArrayForFile()` shall be added to the `CLIEngine` class.
+By providing such a feature people can easily watch the correct files for changes as soon as the configuration changes.
+
+***Example:***
+```js
+import fs = require("fs");
+
+let engine = new CLIEngine({});
+let file = "./App/src/index.ts";
+let configArray = engine.getConfigArrayForFile(file);
+
+for (let config of configArray) {
+    if (config.filePath.length > 0) {
+        fs.watchFile(
+            conig.filePath,
+            () =>
+            {
+                lint();
+            });
+    }
+}
+
+function lint() {
+    engine.executeOnFiles([file]);
+}
+
+lint();
+```
+
+## Documentation
+
+I think it might be a good idea to blog about this new feature as this is a key feature for writing language servers for IDEs.
+That way the integration of `eslint` in all sorts of text-editors can be improved to its best.
+
+## Drawbacks
+
+The only downside I can think of is that on every change to the `ConfigArray` or `CascadingConfigArray` API either a new major version has to be released or a separate implementation of said classes has to be added for the public API.
+
+## Backwards Compatibility Analysis
+
+Adding this feature provides full backwards compatibility.
+
+## Alternatives
+
+Add a method which only returns filepaths and a value indicating whether said filepath is `root`.  
+That way the drawbacks could be fixed but the developers don't have the benefit to further analyze the config-array.
+
+A similar method already exists: The `CLIEngine#getConfigForFile`-method. Though this method does not indicate which files are considered by `eslint` which draws developers of language-servers unable to hot-reload `eslint` as soon as a config-file changes.
+
+## Open Questions
+
+  - Is it reasonable to return the whole config-array or should such a method only return the `filePath`s?
+  - Does it make sense to also return the `root`-directory in order to let language-servers run `eslint` with `root` as its `cwd`?
+  - Is there some more key information other than `filePath` which should be returned? (Such as `name`, for instance)
+
+## Help Needed
+
+I think it should be pretty easy to implement such a feature as the only thing that is required is a simple call to `CascadingConfigArrayFactory#getConfigArrayForFile` and, if required, some data extraction/manipulation.
+
+## Frequently Asked Questions
+
+<!-- - No FAQs added yet --> 
+
+## Related Discussions
+
+ - https://github.com/eslint/eslint/issues/12631