Add "Open Brackets Source" functionality

This is for #8795. It adds Debug > Open Brackets Source, which opens the Brackets source code in Brackets.
It's the easiest possible solution - it just opens the symlink dir (like `C:\Program Files (x86)\Brackets\dev` on Windows).
The menu entry is only enabled in development builds and not in experimental (release) builds.

Since I heard of several problems with symlinks, it's no problem to me if you don't accept this PR.