Support ISpanMappingService for GoTo Definition/Implementation and rename requests

[ajaybhargavb]

Currently FAR and CodeLens use the ISpanMappingService to allow Razor to remap its background C# documents back to the original Razor document. See here and here.

We want Go to definition/implementation and Rename requests to also invoke this API. This will allow the LSP powered Razor editor to provide cross-file navigation in both local and online scenarios.

cc @dibarbet @jasonmalinowski @NTaylorMullen

[dibarbet]

accidentally closed. re-opening for rename support