Allow creating/registering block variations from the UI

Allow registering new block variations and style them without leaving the editor.

Idea:

A theme can register new buttons variations such as pill, rounded, circular and style them to be used in the editor.

Pill	Rounded	Circular

Currently it is not possible to create such variations from the editor.
A block variations can be registered with javascript

const { registerBlockStyle } = wp.blocks;
registerBlockStyle('core/button', {
  name: 'pill',
  label: _x('Pill', 'block style', 'theme-name'),
});

this creates a new button variations and adds a css class wp-block-button is-style-pill which can be styled from the editor global styles.

Proposal:

Create block theme can have a UI to create a new block variation, (which adds the required javascript code and enqueues is safely).

No files, tests, or entry points are named. Start by mapping the existing editor UI and block-variation registration flow, then define the UI, generated JavaScript, enqueueing, and styling behavior. Done means a theme author can create and style a variation from the editor without leaving it.