Part of #9519
Needs dotnet/roslyn#74548 before it will compile
Needs
https://devdiv.visualstudio.com/DevDiv/_git/VSLanguageServerClient/pullrequest/567229
before it will work in VS

There were a few side quests on this one:
* Roslyn OOP, at least the way we access it, doesn't have any options
set, so took a few tries to get the Roslyn side of this right for our
needs
* I wrote this feature test-first so only discovered the lack of options
after I modified Roslyn and our test infra to allow us to set global
options, so ended up removing most of that code, Kept the bit about
isolated workspaces because it just makes sense.
* Had to re-write one of the `DocumentMappingService` methods to use
`TextChange` instead of `TextEdit` so I could use Roslyn LSP types
* The hacky, and fortunately temporary, way we were doing generated C#
content was causing cache misses in Roslyn in tests, so fixed that up
* When I finally got up to manual testing I found a bug in the platform
that meant inlay hints just don't work with dynamic registration, so
filed the above linked PR to fix that

If reviewing commit-at-a-time please note that the first commit was
written before the reworking of extension methods and LSP types, and the
fixes for that are in the last commit.