feat: Introduce BrowserFetcher class (#1983)

This patch introduces `BrowserFetcher` class that manages
downloaded versions of products.

This patch:
- shapes Downloader API to be minimal yet usable for our needs. This
  includes removing such methods as `Downloader.supportedPlatforms` and
  `Downloader.defaultRevision`.
- makes most of the fs-related methods in Downloader async. The only
  exception is the `Downloader.revisionInfo`: it has stay sync due to the
  `pptr.executablePath()` method being sync.
- updates `install.js` and `utils/check_availability.js` to use new API
- finally, renames `Downloader` into `BrowserFetcher`

Fixes #1748.

utils/doclint/check_public_api/index.js

@@ -22,7 +22,6 @@ const Message = require('../Message');
 const EXCLUDE_CLASSES = new Set([
   'CSSCoverage',
   'Connection',
-  'Downloader',
   'EmulationManager',
   'FrameManager',
   'JSCoverage',