codeintel: add support for find-implementations

Created by: chrismwendt

This adds find-implementations to the precise code intel backend. This is part of https://github.com/sourcegraph/sourcegraph/issues/24479

Walkthrough of the feature and code changes: video 🎥 I forgot to cover query_references.go in the video, but it contained mostly refactors like inlining pageReferences and renaming cursor fields.

TODO

• Create an implements graphql resolver
• Implement the bulk of the backend changes
• Implement find-references in the LSIF backend
• Wire implementations through the extension host
• Update https://docs.sourcegraph.com/extensions/authoring/context_key_expressions#available-context-keys
• Support cross-repo find-implementations
• Add tests
• Make sure a context key is set when there is a definition so that the extension can use it in the when expression to toggle the "Find implementations" button
• Fix nondeterministic LSIF diff test