Deprecate theme_get_setting()

Working on this, fixed #36

I think the main thing here is the service/class name. Otherwise this looks good to go.

With regards to the param types. Part of me wants to say yes lets do it and knock it out. But other part is concerned how much it could expand the scope?

no one else had a vote, lets give it a shot.

Done

Believe that was the last bit.

I think we should consider improving the architecture rather than injecting yet another service into the theme installer. I think we could even remove the clear cache method if we listen to configuration events.

Improved the solution substantially with:

1. Config subscriber as requesting by @alexpott to clear theme settings caches when core.extension:theme changes
2. Memory cache service following recommendations in 🌱 [policy] Standardize how we implement in-memory caches Needs work

Some things are missing in the CR and the proxy class likely needs to be regenerated.

Also had a question on the new service.

Addressed feedback, service names are personal preference I guess? Personally not a huge fan of a class name service and interface name alias.