Auto install bundle

Automatically install extensions "bundled" with installer on startup. This feature relies on the installer putting **zip files** for "bundled" extensions into the `[install-dir]/auto-install-extensions/` folder.

This is for #9233 

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### Testing Matrix for `auto-install-extensions` folder

#### Should fail silently
* is missing
* is empty
* contains file which has non-zip file extension
* contains file which has non-extensions with zip file extension
* contains extension that is already auto-installed
* contains extension that is already manually installed
* contains extension that has a newer version auto-installed
* contains extension that has a newer version manually installed
* contains extension that was already auto-installed, then manually removed

#### Should install extension
* contains extension that is not yet installed (auto or manually)
* contains extension that has an older version auto-installed
* contains extension that has an older version manually installed

@dangoor What if `ExtensionManager` listens for `AppInit.appReady()` (instead of calling it from **brackets.js**) ? This way it does not need to be exported at all.