Add "has storage access" boolean to environment

[bvandersloot-mozilla]

This is a concept that originated in the Storage Access API, where it has been stuck because of spec issues between Fetch and 6265bis. To un-logjam this, I've started whatwg/fetch#1807. That depends on this bit existing.

This patch adds the bit, which remains false, and does nothing.

• At least two implementers are interested (and none opposed):
  • Mozilla
  • Apple? (@annevk)
  • Google? (@johannhof )
• Tests are written and can be reviewed and commented upon at:
  • change is not observable, not needed?
• Implementation bugs are filed:
  • change is to update spec interfacing, not needed?
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed:
  • I don't think this is needed?
• The top of this comment includes a clear commit message to use.

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

[johannhof]

To clarify, is this change integrating https://privacycg.github.io/storage-access/#ua-state into HTML? cc @cfredric

If so, that seems fine by me, but I wonder if it should be part of a larger port of the storage access API (🍾 ?)