Migrate away from two Language Servers to power Razor C# / HTML scenarios

[NTaylorMullen]

Today Razor utilizes two language servers to power C#/HTML scenarios. This dual scenario has several perf and reliability implications.

Ultimately this model moves us more towards: https://github.com/NTaylorMullen/LSPVirtualDocuments/blob/master/Documents/EmbeddedLanguageLSPExpansion.md

• [x] #6650
• [x] #6651
• [x] #6652
• [x] #6653
• [x] #6654
• [ ] #6655
• [x] #6656
• [x] #6657
• [ ] #6296
• [ ] Delete HTML C# language server entirely (needs an issue once we get here)
• [ ] #6978