feat: Autocomplete via propTypes, auto close tags, quotes and braces

[markdalgleish]

This adds initial autocomplete via CodeMirror's show-hint and xml-hint addons. This works because the jsx mode internally switches to xml mode when writing JSX tags.

We generate the set of tags, prop names and supported values from the propTypes object thanks to parse-prop-types. Right now, when suggesting values, this only supports string values that were defined with PropTypes.oneOf([...]), but it's much better than nothing.

In the prop suggestions, we're deliberately omitting children (since they shouldn't be provided as attributes) and className (since Playroom is designed to work with components, not style sheets).

Also, to help streamline the editing experience, we're now automatically closing JSX tags, quotes and braces, and converting Tab key presses into spaces.

[markdalgleish]

Just merged from master and fixed conflicts. Ready for review 👍🏻