lsifstore: add ability to query API docs search index - take 2
Created by: slimsag
I had previously sent these exact commits in https://github.com/sourcegraph/sourcegraph/pull/26487 but as I learned more the PR grew, and I wanted to land more backend tests first, so I closed it and sent:
- https://github.com/sourcegraph/sourcegraph/pull/26736
- https://github.com/sourcegraph/sourcegraph/pull/26913
- https://github.com/sourcegraph/sourcegraph/pull/26954
Now that those are merged, it's time to land this.