search: interpret escaped parentheses in heuristic

Created by: rvantonder

DO NOT REVIEW, WIP. Description is for myself.

Stacked on #11128.

This PR changes a function that controls a heuristic when interpreting syntax like () as patterns versus expression groups. When this heuristic encounters escaped parentheses, like \(\), it will interpret these (including slashes). We want to actually interpret this escape sequence here, since if this heuristic fires, it implies that the string is literal.