lsifstore/apidocs: optimize and improve TextSearchQuery, RepoSearchQuery, etc. logic

Created by: slimsag

This includes various improvements to the (currently unused) apidocs package to optimize and improve behavior, including:

• Adding query parsing logic to "pick out" possible repository names from queries, so that we can be strict about what we match against the repo name table and avoid false-positives, allowing us to actually limit our search space to specific repos if they match query terms - instead of just ranking possible-repo-name-matches higher in the result list as the prior approach tried to do.
• Massively reduced :* tsquery prefix operator usage in TextSearchQuery, as it's very slow. We're now very cautious about when and where we utilize it.
• Prevent emitting duplicative OR conditions as we previously did.
• Prevent emitting empty WHERE conditions () (illegal SQL) when there are no query terms.
• Made WHERE conditions always wrapped in ( parens ) so that we don't need to worry at the usage site where we sqlf.Sprintf("WHERE %s AND ...", condition) them in.

All changes have tests, and added new tests in some cases to catch edge cases I hadn't previously thought of.

Helps #21938