Update export options

[g-elwell]

Description

This PR improves the theme.json export by excluding core WP styles that shouldn't go into the theme.json file. It also adds a new option to export only the user customisations that have been made, which can be useful if you want to store these as a style variation on top of a base theme.json.

Change Log

• Extends our REST API export endpoint, to support which type of theme data is included in the resulting output
• Adds additional export option for exporting customisations only

Steps to test

• Activate the plugin up on a theme which has a theme.json file
• Make some customisations to the theme styles and save
• Try the 'theme.json' export, all data should be exported
• Try the 'customisations only' export, only your customisations should be exported

Screenshots/Videos

N/A

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules