Make VSCode LiveShare work without errors about razor

[NTaylorMullen]

Some investigations playing around with VSCode LiveShare:

• VSCode Insiders -> VSCode completion doesn't work. Throws workspace null on guest
• VSCode -> VSCode Insiders. Throws workspace null on guest but otherwise works
• VSOnline -> VSCode Insiders provisional completion doesn't work. Throws workspace null on guest
• VSOnline -> VSCode provisional completion doesn't work. Throws workspace null on guest

[NTaylorMullen]

Talked with LiveShare team about the workspace being null on the guest and it is definitely unfortunate but after the initialize request no other LSP requests should be sent to the guest LSP. Therefore, we should do the work to make our workspace resolution logic not explode but more importantly short-circuit the remaining file change detector initializations in response.

[NTaylorMullen]

Talked more with LiveShare folks and for our client side functionality such as auto-completing HTML tags and provisional C# completions they suggested it might be wise for us to restrict what type of Razor files we operate on in the client by using document selectors on the client that only understand the file:// scheme (LiveShare uses the vsls scheme).

[NTaylorMullen]

Looks like in the latest release LiveShare stopped working for all sorts of completions.