Module Blocks

[surma]

DO NOT MERGE YET

This PR adds the minimal integration of the TC39 Module Blocks proposal (spec) to the HTML spec. Specifically:

• Adds support for Module Blocks to import()
• Handles Module Blocks in the module map
• Adds special cases for Module Blocks to StructuredSerializeInternal and StructuredDeserialize
• It does not add support for Module Blocks to the Worker constructor, as we are still settling on a way forward here.

I got a bit caught off-guard by the changes to the module map with import assertions. I think I resolved the conflicts correctly, but please be extra thorough when looking at that part.

cc @littledan

Checklist:

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chrome: …
  • Firefox: …
  • Safari: …

(See WHATWG Working Mode: Changes for more details.)

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