This adds navigateEvent.prototype.intercept({precommitHandler}).

Instead of always "committing" the navigation immediately, the navigation only commits after all its precommitHandler promises are resolved.

In addition, a precommitHandler receives a NavigationPrecommitController as an argument, and can use it to change
the URL of the ongoing navigation before it is commited, using the redirect(newURL) function.

See https://github.com/WICG/navigation-api/blob/main/README.md#precommit-handlers

• At least two implementers are interested (and none opposed):
  • Chromium: implementing
  • WebKit: Navigation API: deferred commit WebKit/standards-positions#449
  • Gecko: Navigation API: deferred commit mozilla/standards-positions#1169 (comment)
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/navigation-api/precommit-handler?label=master&label=experimental&aligned&q=precommit
• Implementation bugs are filed:
  • Chromium: mostly done, opened https://issues.chromium.org/issues/422067785 for the last bit.
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1970123
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=293952
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A, no HTML elements involved in this
• MDN issue is filed: Will be done as part of Chromium beta launch
• The top of this comment includes a clear commit message to use.

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

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