Try: Add custom patterns to theme filesystem on save

[mikachan]

WIP

A PR to explore the most minimal way we can introduce some pattern management into the plugin by allowing custom patterns to be saved to the theme filesystem on save, allowing them to be exported with the theme.

Part of #287.

Testing Instructions

1. Create a new pattern in the Editor (synced or unsynced, also try adding categories)
2. Go to the Editor, and in the Create Block Theme menu, save the theme changes with the "Save My Patterns" option checked