* Rename TypeDefinitionProvider to TypeImeplementationProvider

**bug**
In #18346, I originally called the new go to implementation provider api `ImplementationProvider` which we then decided to rename to `TypeDefinitionProvider`. At the time, I didn't realize that a type definition was actually its own, unrelated concept.

**Fix**
Rename `TypeDefinitionProvider` to `TypeImplementationProvider` to make it clear what the purpose and use of this api is.

* Fix a few names in comments