Add ability to switch on routes (like Theme Change, but with wildcards)

Problem/Motivation

Cannot switch theme based on routes, unlike the much less-maintained https://www.drupal.org/project/theme_change/ → module

Steps to reproduce

n/a

Proposed resolution

Add theme switching by routes, allowing for wildcards.

Remaining tasks

User interface changes

Add new category in list of "conditions", modeled on path, except that it allows a list of routes to switch on (including wildcards)

API changes

Data model changes