Add basic import maps support

This imports much of the specification from https://wicg.github.io/import-maps/. The main deltas are:

* Does not include support for external import maps. This allows a good deal of simplification, such as by changing "acquiring import maps" + "pending import map script" + "wait for import maps" to a single "import maps allowed" boolean, or integrating "register an import map" into "execute the script element". If a src="" attribute is present on an importmap script element, then an error event is fired. See #8355.

* Does not include any support for non-Window import maps. The spec draft included stub support for those, with other globals always having an empty import map. This instead ties import maps to Windows directly. If we revisit that in the future we can refactor, but keeping always-empty import maps around on the other globals was clumsy.

* Adds introductory text and examples.

* Adds conformance requirements, both on the script element and on the import map JSON.

* Fixes a few minor specification issues: WICG/import-maps#267, WICG/import-maps#268, WICG/import-maps#270, WICG/import-maps#276, WICG/import-maps#277.