[form-builder] Expose the withDocument HOC

[bjoerge]

This exposes a higher order component that makes the current document available for any component in a form-builder context. By default, input components are not given the document, but there are cases you might want it, e.g. if you are a slug input that depends on another document field.

Example usage:

import {withDocument} from 'part:@sanity/form-builder'

function MyInput(props) {
  return (
    <div>
      Document title: {props.document.title}
      {/* ... */}
    </div>
  )
}

export default withDocument(MyInput)

[rexxars]

👍