[LinkTagModuleImport] render blocking

Some people want to solve FOUC with module <link> stylesheet tags.

I didn't see it in the explainer, so I thought I'd comment that I think it should be non-blocking by default, with opt in:

Non-blocking:

<link
  rel=stylesheet
  type=module
  href=foo
/>

Blocking (one way to avoid FUOC with the trade off that content after the link tag is not rendered until after the network request):

<link
  rel=stylesheet
  type=module
  href=foo
  blocking
/>

Start by reading the LinkTagModuleImport explainer referenced in the issue and compare its treatment of module stylesheet links with the proposed examples. Determine whether non-blocking behavior should be the default and how an opt-in blocking attribute would be specified; done means the explainer clearly records the chosen semantics and trade-offs.