Add ability to customize block settings and defaults via theme.json

[mrwweb]

I often want to hard-code one or more of the settings this block provides. In those cases, it would be nice to allow removing the various settings from the editor interface. I think I've seen examples of this done through theme.json in settings.blocks.mrw\accordion so, for instance, I could either set a default icon that's different from the plugin default OR remove the icon setting altogether.

See also: #5 for thinking about custom expand/collapse icons

[mrwweb]

Closing. This is what block variations are for, especially now that they can be made in PHP or JS!