Add external import maps

[annevk]

@Constellation implemented external import maps in WebKit as per https://wicg.github.io/import-maps/. They're not included in #8075, but still seem worth standardizing.

cc @domenic @hiroshige-g @allstarschh

Issues we need to triage as part of this:

• application/importmap+json MIME type registration WICG/import-maps#163 + Official file extension? WICG/import-maps#282 (tl;dr: register MIME type with IANA)
• External import maps support WICG/import-maps#235 (although this just seems like a Chrome bug report maybe?)