Backend work for regex mode toggle for Zoekt code search

Problem to solve

Currently the in Zoekt code search UI, there's no way to switch between a literal search mode and a regex search mode. We support both modes today.

Proposal

• Add a toggle to the search bear when in the code scope that allows the user to select regex or literal search modes:

• Persist the user's selection so that next time they run a search, that last mode used is used