search: capitalize operators in human query strings

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/36761.

As in title, cosmetic change. It's more obvious that operators like AND have special meaning when capitalized in alerts as introduced in https://github.com/sourcegraph/sourcegraph/pull/36761, because we don't highlighting those queries.

Test plan

Updated tests.