Core: Implement API sets multitargeting for CSharp

[NTaylorMullen]

Today we rely on a massive contextual object that contains strings used at code generation time. This has been a very painful way to enable code generation extensibility in Razor; should improve on it. 💪

[NTaylorMullen]

When we tackle this feature we should also take a look at DefineSection since it'd be nice to have its signature be string, Action; instead string, Action<T> (needed for back compat). This is all to enable us to not bring forward WriteTo, WriteLiteralTo etc.