[Cohost] Improve OOP serialization of Html documents

[davidwengier]

Right now we transfer the contents of Html virtual documents from OOP, where they are computed, and send them back to the IDE as a string so we can make an LSP call. We should see if we can expose something like Roslyn's SerializableSourceText to make this more efficient, and avoid LOH allocations.

The other option is to not go to OOP for the Html document, but rather see if it's possible to create a non-allocating fast parser in the IDE that can produce it. Since it doesn't require semantics, it can probably be done direct from the Razor source which is already in the IDE at the point of the OOP call.