search: interpret // specifically as a literal pattern for regexp search

Created by: rvantonder

Our regexp search syntax includes delimited slashes, like /<thing>/ to search for a regexp <thing>. This is true even when <thing> is empty. So, we will convert // to "" (empty string) when the pattern is a regexp.

There's very little utility of searching for "". It is far more likely that a user would intend to search for // comments when the value is empty. So I support the behavior for // to be interpreted literally in regexp mode. The main thing instigating this change is the previous unexpected behavior when // means empty string, and seeing unintuitive results, esp for code insights matching //-style comments: https://github.com/sourcegraph/sourcegraph/issues/31195#issuecomment-1043352730

Test plan

Adds unit tests for new behavior.