JSX support

Merged username-removed-383729 requested to merge jsx into master

By popular demand...

This implements very basic support for JSX. Still to do:

  • Support pragmas other than React.createElement
  • Optimisations (see here and here)
  • Some method to auto-import needed modules? (e.g. automatically add import * as React from 'react'

It's still an open question whether this truly belongs in core, but I do like the convenience of it.

Since I don't ever use JSX, I could have completely pooched this up – would welcome input from people more experienced with it.