Add template states to the Site Editor

[Aljullu]

In the frontend, there are many variables that might impact the way some templates or blocks behave. For example, the Search Results template might look different depending on whether there are results or not.

This issue is about providing a mechanism to enable those "states" in the Site Editor, so users can preview how the templates will look like in the frontend under certain circumstances.

Ideally, this should be extensible, so plugins can register custom states and have their own blocks adapt to those "states".

This is related to providing a system to preview templates with certain data (#28466), but it differs in that these templates states are not directly attached to an entity.

It's also related to block states (#57719 and #38108), the difference is that template states impact the entire template or several blocks instead of only one.