Go To Source Definition UX improvements

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Following up on TypeScript’s new Go To Source Definition command (d851ea5d49b0ec89b98765d9e0084c25f2ff5031), I want to track some editor-side UX improvements: - Can we replace the error notification for “no source definitions found” with an inline message similar to Go To Definition? <img width="330" alt="image" src="https://user-images.githubusercontent.com/3277153/173925638-1fcba3e2-5feb-40cd-9488-3159fc19999a.png"> - Can we somehow indicate when a result is a guess? This is already returned in the protocol response via an `unverified` property, but it is never surfaced to the user. When multiple locations are returned, maybe we could add something to the peek locations UI: <img width="611" alt="image" src="https://user-images.githubusercontent.com/3277153/173927129-0943cadc-85c6-4090-b8d1-c1f7f7441b37.png"> (Note: the `unverified` flag is also sometimes set in normal Go To Definition responses, so this request is not specific to Go To Source Definition.) When only one location is returned, maybe a notification would suffice, but I would love design input on this.