Motivation In this PR #29335 I tried to get the "layout" setting from theme.json and I had to call 4 functions with the right arguments in order to do it.

While working with the Theme config abstraction in the backend, it became very clear to me that the classes and APIs were built initially to map the theme.json file and format, but they were too low level to work with which resulted in a lot of indirection and duplication each time you want to access a config or something in the backend.

This PR makes a step into simplifying the APIs by introducing a WP_Theme_Config object that you can access with:

// Gets the config from the cache or load it
$config = WP_Theme_Config_Resolver::load_config();

// Get settings
$config->get_settings();

Notice that I didn't have to pass any argument or anything, everything is abstracted from the consumer, making it easier to reason about the current config without actually knowing how it's assembled, what kind of theme it is...

Later, I'd like us to probably introduce a function similar to the useEditorFeature hook we have in the frontend.

Also note that in the PR, I keep the WP_Theme_JSON class but in reality this class becomes just a private implementation detail that shouldn't be accessed directly at all. A future refactor might remove it entirely.