* Initial work on signature help context

Fixes #54972

Adds `SignatureHelpContext`. This tells providers why signature help was requested

TODO:

- [ ] Better understand semantics of retrigger. Should `retrigger` be an flag instead of a `triggerReason`?
- [ ]  Fix skipped test
- [ ] Add more tests for trigger reasons / trigger characters

* Fix unit test

* Make sure we retrigger sig help if it is already showing

* Add test for dismiss and re-invoke

* Extract some constants

* Extract createMockEditor

This reverts commit 315cce83.

This reverts commit 1c72ab24.

we keep the LogLevel-enum and the plan is to accept the log level when
calling OutputChannel#appendLine so that
logging is supported there

to avoid breaking live share in Insiders

Finalize the definition link api

- Gives fields more explicit names (target and origin)
- Moves api to vscode.d.ts
- Makes other definition providers (such as type definition provider and implementation provider) also return definition links

Fixes #54101