Don't load Razor when not necessary

[davidwengier]

With cohosting, we're in Roslyn's LSP server which loads for all Roslyn projects. We used to only load for the existence of .razor files (through IDynamicFile) or Razor files being opened (through TextViewConnectionListener/ContentType). We should make things smarter and only do work when we need to, using UI contexts.

See https://learn.microsoft.com/en-us/visualstudio/extensibility/how-to-use-rule-based-ui-context-for-visual-studio-extensions?view=vs-2022 and a useful search

Long term, we probably want this on our package, but we can at least start with the cohosting dynamic registration service.