Gives our generated a files an extension that isn't used for any other purpose (that we know of). This is handy for tooling to be able to quickly know if a file is 'ours'. This comes up in places like IVsSymbolicNavigationNotify (go to definition).
Fixes #1986
* Use File.Open overload that overwrites file entirely in DiscoverCommand Re-enable TagHelper discovery incrementalism test Fixes #2104