Navigation API: add deferred commit functionality

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 committed, using the redirect(newURL) function.

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

Closes #10919 by superseding it. Maybe closes #10716. Closes WICG/navigation-api#285.

Author: noamr