Widgets that output list of links can now be wrapped within `<nav>` elements to improve semantics and accessibility.
The `<nav>` elements are also native landmark regions, which helps assistive technology users to navigate through them. Themes can opt-in to this new behavior by declaring support for the new `html5` feature `navigation-widgets`.
Props joedolson, simonjanin, audrasjb, afercia.
Fixes#48170.
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