Boost exact matches in suggestions

This orders the suggestions into five groups:

1. Exact matches in brackets, e.g. when searching for "foo", `Bar (foo)` will be shown first.
2. Exact prefixes in brackets, e.g. `Bar (foo-bar)` (yes, this is a little optimization for the special case of languages with sub-languages).
3. Prefixes, e.g. `Fooxyz (xyz)`.
4. Prefixes in brackets, e.g. `Bar (fooxyz)`.
5. Whatever is left, e.g. `Xyzfoo (xyz)`.

Ordering inside the groups will be alphabetical.

[Bug: 67395](https://bugzilla.wikimedia.org/show_bug.cgi?id=67395)