* 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