lsifstore: refactor API docs text search utilities into separate package

Created by: slimsag

Keeping lsifstore logic more isolated, as I add querying logic there's a bit of complexity for composing queries against the DB (including this code for lexeme management, etc.) that is nice to keep isolated in a separate package.

Helps #21938

Signed-off-by: Stephen Gutekanst stephen@sourcegraph.com