Better grouped-expression heuristics

[rvantonder]

Usability of and/or queries is hampered by tight binding to filters. The query:

repo:^github\.com/sourcegraph/sourcegraph$ (a and b) or (c and d)

needs parentheses:

repo:^github\.com/sourcegraph/sourcegraph$ ((a and b) or (c and d))

as the hoist heuristic is not sufficient for more nested expressions. For future nested searches, the interpretation

(repo:^github\.com/sourcegraph/sourcegraph$ a and b) or (c and d)

is indeed what we want, but this is arguably less common than the former example. The right move might just be to remove tight binding to fields.

[rvantonder]

An ideal solution would be to have a "showing results for..." with a parenthesized query according to some heuristics, that can be changed.

[rvantonder]

Similar to examples in #9816

[rvantonder]

All reasonable heuristics are implemented in previous iterations, closing.