search: standard query interprets literal and regexp patterns
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/37655.
This lets standard
queries interpret /<regex>/
as regex and literal otherwise. The parser stuff probably needs a bit of tidying at some point, but even better would be if we just remove regex and literal parsers
Test plan
Adds tests. Still unused.