JSON module support

This commit adds JSON modules as a single default export, with parse
errors checked before instantiating the module graph.

As infrastructure, this divides the "module script" concept into
"JavaScript module scripts" and "JSON module scripts". Most of the
spec's existing uses of "module script" become "JavaScript module
script".

JSON module scripts are fetched in the same way as JavaScript module
scripts, e.g. with the "cors" mode and using strict MIME type checking.
They use the Synthetic Module Record defined in
whatwg/webidl#722.

Closes whatwg#4315.
Closes WICG/webcomponents#770.