Support React form fields

[markspolakovs]

With this, you can use a React (well actually Preact) component as a MyRadioFormField semi-seamlessly.

Caveats:

• It uses ES Modules, so no bundling/minification is performed on the field file
• It uses Preact rather than React (because React doesn't properly support ESM)
  • TODO: add preact/compat so we can use all React libraries (need to figure out how on earth preact/compat works with ESM)