✨ Add support for route to test CodeLens

[savannahostrowski]

Closes #142
This PR adds support for route to test code navigation via CodeLens (basically the complementary feature to the existing test to route code navigation). Some additional details:

• No Code Lens will show on a route that has no tests (vs. showing "0 tests", which felt needlessly noisy to me)
• The design here is such that we scan the workspace on session start but then incrementally invalidate files as a user edits to prevent whole workspace re-scan.
• This also adds a type field on the property bag for the existing telemetry event that tracks whether we were able to provide a CodeLens.
• This new feature is bucketed under the existing codeLens.enabled setting. I've opted to not carve out a separate setting for this as I suspect that users who don't want test to route code nav will probably also not want route to test code nav.
• I extracted findTestClientCalls from TestToRouteCodeLensProvider into extractors.ts so it can be shared with the new test index.