Support interpolations in C# strings

[michaelblyons]

We embed C# when a heuristic is met in here-strings. But @"..."@ here-strings should still support PowerShell interpolation even if there's C#. It may not be used all that often, but we should still note it.

See #213 (comment)

[michaelblyons]

Thank you to deathaxe for adding a string_protoype context in sublimehq/Packages#4484. That should make this somewhat simpler on the PowerShell end.