allow inline mdx fields in custom content components

[stefanprobst]

it would be very cool if the following schema was supported:

content: fields.mdx({
  label: "Content",
  components: {
    Widget: wrapper({
      label: "Widget",
      schema: {
        title: fields.text({
          label: "Title",
        }),
        summary: fields.mdx.inline({
          label: "Summary",
        }),
      },
    }),
  },
}),