API docs: implement LSIF extensions for Go LSIF indexer / langserver

Created by: slimsag

Depends on:

• API docs: scope out initial project implementation (#19385 (closed))
• API docs: define rough user experience, workflows (#19386 (closed))
• API docs: figure out exact data requirements from LSIF (#19387 (closed))
• API docs: propose LSIF extensions (#19388 (closed))

This will involve actually implementing and landing the extensions we propose in the lsif-go codebase https://github.com/sourcegraph/lsif-go

I have not looked at that codebase, but anticipate this will take ~3 weeks approx.