Use Roslyn project info in cohost endpoints

[davidwengier]

Part of #9519

This PR is the very first baby step towards using the Roslyn project system in cohosting.

What it does:

• Creates document and project snapshots from Roslyn document and project snapshots for document color and semantic tokens requests
• Provides a point for @chsienki and co to put their calls to the source generator to get the generated code (in CohostDocumentSnapshot.GetGeneratedOutput) and host outputs (various places in CohostProjectSnapshot)

What it doesn't do:

• Anything good for performance. This is blindly discovering tag helpers whenever a project snapshot changes, rather that trying to carry them forward
  • The source generator will do that work
• Remove any of the old systems, meaning everything is running in parallel with the current Razor project system
  • As can be seen with the hacky method on DocumentVersionContext where the new code just blindly assumes the latest version, because document snapshot generation during typing is still coming from the Razor project system

Very much a stepping stone. Next steps is to work out how to get RazorCustomMessageTarget onto using roslyn types, or remove it, or just skip synchronization if cohosting is turned on.

[DustinCampbell]

Consider using a PooledArrayBuilder if the result will regularly be larger than 4 items.

[davidwengier]

Do you mean if it will be less than 4 items? If it's more than 4, doesn't the PooledArrayBuilder just switch to using the ArrayBuilderPool anyway?

[davidwengier]

Having said that, I just copied this code as is, and it can be improved slightly by specifying a capacity at least

[DustinCampbell]

Yes, sorry - I meant "less than 4 items". PooledArrayBuilder can help avoid starvation of the ArrayBuilderPool and is useful when smaller arrays are expected. I'm not 100% sure what GetRelatedDocuments means. Based on the code and the name of the map, I would guess it means, "the documents that use this import"? If so, ArrayBuilderPool is probably the way to go.

[davidwengier]

Yep, that's my reading too, and I agree. Thanks for confirming