Add the "speculationrules" destination

[domenic]

Part of whatwg/html#11426, for requests initiated by the Speculation-Rules HTTP header.

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This should be merged a bit after whatwg/html#11426. Otherwise it will reference the WICG draft.

• At least two implementers are interested (and none opposed):
  • See Add speculation rules prefetch html#11426
• Tests are written and can be reviewed and commented upon at:
  • This is tested by all tests of the header: https://github.com/web-platform-tests/wpt/blob/c669bcf970fe50b56e0ee53de9c2097a012dafc7/speculation-rules/prefetch/resources/ruleset.py#L40-L43
• Implementation bugs are filed:
  • See Add speculation rules prefetch html#11426
• MDN issue is filed: "speculationrules" request destination missing mdn/content#40289
• The top of this comment includes a clear commit message to use.

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

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