HTML script element support for type=importmap

[hamishwillee]

FF108 adds support for "import maps", a mechanisms for providing a mapping between a module name and its location when you're importing a module. The feature is implemented using an HTML script element that has type="importmap", where the element contains the import map as a JSON definition.

Compat:

• Firefox 102 - 108 implemented behind a preference: https://bugzilla.mozilla.org/show_bug.cgi?id=1688879
• FF108 ships: https://bugzilla.mozilla.org/show_bug.cgi?id=1795647
• Chrome 89: https://chromestatus.com/feature/5315286962012160
• Safari - not implemented (see chrome signal, and canIuse).

Fixes #17701

Other docs work can be tracked in: mdn/content#16150

FYI @queengooborg

[queengooborg]

LGTM, but let's remove the flag data -- since Firefox 108 is coming out soon, we're going to remove it soon anyways!

[hamishwillee]

Thanks @queengooborg -ready for re-review