Add a ThemeRegistryAlter attribute

Problem/Motivation

Sometimes there are theme hooks/templates provided by core/other modules where we might want to sub in a design-system template instead of writing a thin wrapper template.
In such cases it would be neat if we could add a #[ThemeRegistryAlter('{theme hook}')] to enums to have their path/libraries registered as the path for a template/hook provided by another module.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes