Sync up language-csharp with work on other derivations

[damieng]

Brings in changes from some other repos that have continued work on the C# syntax.

Fixes

• String interpolation: fixes #43 (supersedes and closes #49)
• Nullable types: fixes #58
• Not treating keywords with @ in front of them as keywords (in some places)

Improves

• Generic type parameters and constraints for type declarations
• Property, field and variable declarations

[jamesqo]

Hooray! It's been so long since interpolated strings were introduced, nice to finally see them supported.

[damieng]

If you want to try it out today before it gets integrated with atom you can;

apm dev language-csharp

then cd into the github/language-csharp folder and type

apm link (also you can git pull when in this folder to pick up new changes as they are added here)

then launch atom and let me know what you think. If you have any problems and want to revert back just type

apm unlink language-csharp to go back to the baked-in version.