Simplify CapabilityRegistry

Improve `CapabilityRegistry` and surrounding code in a couple of ways
- Make `CapabilityRegistry` an actor
- Refactor `CapabilityRegistry` to duplicate less code
- Ensure that `SourceKitServer` reports all capabilities of `SwiftLanguageServer` if the client doesn’t support dynamic capability registration. We were missing this for semantic tokens and inlay hits
- Add a few doc comments

Author: ahoppen