[preview] Expose the materializePaths function used in the preview package

[bjoerge]

This exposes a function for materializing a set of paths on a given value. Useful e.g. when you have a partially fetched document but need to retrieve more of its properties, e.g.:

import {materializePaths} from '@sanity/preview'

const partialDoc = {
  _id: 'abc'
}

materializePaths(partialDoc, ['title', 'publishDate'])
  .subscribe(doc => {
    console.log('Title: %s, publish at: %s', doc.title, doc.publishDate)
  })