[LinkTagModuleImport] render blocking #1375

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
/>