Separate schema file for Style Variations

[johnstonphilip]

What problem does this address?

A separate schema file might be needed for style variations. There are already several differences:

• A theme.json schema does not include a "title" field, but it is required for a style variation
• Not all fields get saved to the database even though they exist in the style variation. Those should be reflected in a schema file so people know which fields to leave out of style variation .json files.

What is your proposed solution?

A separate, dedicated schema file should be created if style variations are going to have different supported fields and values.

[Mamaduka]

@johnstonphilip, the theme in this case is the origin of the settings and doesn't need to be defined in theme.json. Instead, these are used for specifications and how styles are merged.