Issue title: Split data implementations
 Issue body: We should have a `data` package, and child packages:
* `data-theme-mods`
* `data-option`
* `data-site-option`
* `data-user-meta`

We can then use hooks where needed to get the proper value and add overrides where needed depending on the option-type.
Right now these are all in the `core` package.

This should also make future implementations (like `post-meta`) easier to implement.
 Comments: 
 Comment 0: Done for `option`. The other option-types are not used anywhere so this one is safe to use as-is for v4.0 and we can expand on the implementation further in v4.1.
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