[meta] Port ThemeKey rules and properties to Drupal's condition system

Problem/Motivation

Meta issue to organize porting of rules and properties to Drupal's condition system, as discussed in #2071685: ThemeKey 8.x Brainstorming / Roadmap → . Plan outlined by @mkalkbrenner:

ThemeKey rules and properties should be migrated to Drupal's condition system that was introduced in Drupal 8. There's already a tiny theme negotiator that does the theme switching based on conditions: Theme Switcher.

We decided to leverage Theme Switcher as new "engine" for Drupal 8 and 9.

So the current plan is to keep Theme Switcher small and to use ThemeKey as an extension to provide additional conditions if required.

Like in the past it would be better if other contrib modules provide their own conditions, just like we encouraged them to provide their own ThemeKey properties. But we still have the freedom to do some stuff here.

List of issues

• ✨ Allow a setting in User Profiles so that users can select a theme from a list. Closed: won't fix