[Features] Features should be configurable outside of the `createApp` helper

[titouanmathis]

Features like breakpoints can be configured today when calling createApp():

import { createApp, Base } from '@studiometa/js-toolkit';

class App extends Base {
  static config = {
    name: 'App',
  }
}

createApp(App, {
  breakpoints: {
    s: '40rem',
    m: '80rem',
    l: '100rem',
  },
});

With the introduction of the registry in v3.4.0 and of the registerComponent() helper, the createApp() function is no longer the only way to mount components.

We should either have:

• one helper function for each feature: defineBreakpoints(), defineAttributes(), etc.
• a single defineFeatures() helper function
• or a Feature component whose options could be mapped to the features map