search: explicitly enclose regex string output in /.../
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/38357.
This brings back https://github.com/sourcegraph/sourcegraph/pull/37659/commits/2ff61220d28eebaf35700b7b8d6b10830491c675.
Since @camdencheek already approved that, and code insights confirmed this doesn't affect usability it should be an easy stamp.
About the question to escape, I've decided to forego doing the escaping /
for the user, since we are robust against it.
Test plan
Updates tests