Extract GitLab UI FormBuilder to a gem

The following discussion from !64260 (merged) should be addressed:

• @tkuah started a discussion: (+4 comments)

  Thanks @peterhegman - this is an awesome start 💚

  Just a few small suggestions/questions from me.

  I think we should try this out in a few places then start extracting this out into its own gem. WYDT ? The gem can live in it's own project (probably easiest in terms of GitLab CI), or live temporarily in vendor/gems

See also !64260 (comment 627492617); perhaps https://gitlab.com/gitlab-org/gitlab-ui is a good place for this to live.