Make the query input in the Code Insight creation UI a real search box, with autocompletion, syntax highlighting and toggles

Created by: felixfbecker

Currently, our search input is just a plain text input, which means users don't get the same kind of assistance they get when using the search box elsewhere in the product. For example, it is not clear how to change regex vs plain matches, case sensitivity, or use structural search. Operators are also not completed and highlighted, and regex patterns are not explained with hovers. Invalid search queries are not highlighted with squiggles.

Current:

Desired:

It would be great if we could use the same search box from the search box in the code insights creation UI page as a reusable component.

cc @sourcegraph/search-product