search: fix field value scanner

Created by: rvantonder

Addresses #12360:

New parser thinks that parenthesized expressions after fields, like file:(...) could mean a start of a grouped expression, and passes off logic to other functions that end up treating it like a pattern. We should assume that parenthesized expressions after fields are never the start of a grouped expression.

Side note: I'll be tidying things in this code to make it more clear how the scanning/parsing works in the next couple of PRs, since there's kinda a lot going on.