Bug 1784812 - Use common JSONWriteFuncs when writing to a string - r=canaltinova,media-playback-reviewers,alwu

Most users of JSONWriter want to fill a string, so instead of having all these
similar implementations, we now have central reusable implementations:
- JSONStringWriteFunc contains a string and writes to it.
- JSONStringRefWriteFunc references a string and writes to it. This is most
  useful when the string already exists somewhere, or needs to be returned from
  a function (so we avoid another conversion when returning).

Differential Revision: https://phabricator.services.mozilla.com/D154618