add uri-property to CallHierarchtItem, better wording, better behaviour when switching calls/callers

- annotate undefined returned types
- Don't call functions with null or undefined values.
- Refactor _convertCompletionItem so that we don't need to mutate the returned object
- Suppress a type error related to electricCharacters.

Const provides better type guards and can make logic clearer

Introduces the concept of a re-trigger character to the signature help provider. This is a seperate set of characters that are registered with the provider. Typing a retrigger character fires a new signature help request if signature help is already showing.

#54972

We compile using the alwaysStrict flag so these directives are not needed

This part removes most `use strict` directives that are right after the file header

* 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

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

* Definition link

Add a new `DefinitionLink` type. This type allows definition providers to return additional metadata about a definition, such as the defining span.

Hook up this new provider for typescript

This PR replaces #48001

* Correctly mark field optional

* Small code fixes

- Use lift
- Remove unused param

* Adding documentation