Provide region_id as a default class for the layout region template

Already passed to the render array, but not defined in hook_theme(). We needed that to add styling based on a specific layout/region.

Note that the removed region.id() comment is not the same, it was empty in our testing, and it is afaik the UUID, not the id/name of the region in the layout definition, as the layout plugin currently doesn't really have an inderstanding of that (only in the configuration, as discussed before)