Performance: Consolidate redundant APIs into getCurrentTheme

[t-hamano]

The theme.json modal and the metadata modal reference their own endpoints to retrieve data that the getCurrentTheme selector can't retrieve, but this is redundant:

• theme.json modal: getCurrentTheme + fetchThemeJson
• metadata modal: getCurrentTheme + fetchReadmeData

It would make more sense to include the additional data in the getCurrentTheme response.

Another reason for submitting this PR is to enable future updates to theme templates and metadata without a browser reload.

If all data is integrated into the default WordPress store, invalidateResolution will allow the selector resolution cache to be refreshed without a browser reload.

[mikachan]

Thanks for working on this, it's looking great! I've left some minor comments.

[mikachan]

I think this is good to bring in, thanks @t-hamano!