Ability to extend fields available on type

[andrewmumblebee]

Is your feature request related to a problem? Please describe.
It would be great if we could add additional fields to the type, similar to how the type: 'image' built-in type works.

i.e.

defineType({
  name: 'codeBlock',
  title: 'Code',
  type: 'code',
  fields: [
	defineField({
		name: 'caption',
		type: 'string',
		title: 'Caption',
	})
  ]
})

Rather than currently where you need to create an extra level of nesting in the object to combine the code fields with other fields

defineType({
  name: 'codeBlock',
  title: 'Code',
  type: 'object',
  fields: [
	defineField({
		name: 'code',
		title: 'Code',
		type: 'code'
	}),
	defineField({
		name: 'caption',
		type: 'string',
		title: 'Caption',
	})
  ]
})

Although this works fine, you lose the built-in preview when using PortableText within the editor. Thanks