FEAT: Add lazy loading support for images

Thanks @akmittal loading is actually really cool and useful but, for now, I'd like to avoid extending core components (this might lead to unexpected behavior if someone creates a new component by extending the image). For anyone who wants to make use of loading attribute can easily extend the image component in this way

editor.DomComponents.addType('image', {
  model: {
    defaults: () => ({
      traits: [
        'alt',
        {
          type: 'select',
          name: 'loading',
          label: 'Loading',
          default: 'eager',
          options: [
            { id: 'lazy', name: 'Lazy' },
            { id: 'eager', name: 'Eager' }
          ]
        }
      ],
    }),
  },
});