Fixes #33935 - Cache setting categories

Every call to `@settings.category` in the index page recalculates all
the category names and labels by iterating over all settings. Since the
categories can't change in runtime, we can calculate it once and store
the list on the settings registry. We can also get the label once per
category instead of repeating it again for every setting. On my
instance, this led to a reduction of about 50k allocations and 20ms.

There are definitly more areas for improvements. Most of the remaining
allocations come from the `grouped_settings` helper, and specifically
inside the `setting.select_values` calculation.