[review] Landing livedev2 in Brackets as an alternative implementation

This is just an initial approach for moving livedev2 extension to Brackets core as per njx/brackets-livedev2#24.
Current implementation was moved to ```LiveDevelopment/impls/default```, livedev2 implementation was added to ```LiveDevelopment/impls/livedev2```. 

They can be switched by a preference named: ```livedev.impl``` which is being checked at ```LiveDevelopment/main.js```.

@busykai,  please review this initial commit to validate overall approach and ```PreferencesManager``` usage. It seems that is not retrieving pref value when manually setting it at project level in ```.brackets.json```