Accessibility: Widgets: Add theme support to make widgets output list of links...

Accessibility: Widgets: Add theme support to make widgets output list of links wrapped within a `<nav>` element.

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.

Built from https://develop.svn.wordpress.org/trunk@48349


git-svn-id: https://core.svn.wordpress.org/trunk@48118 1a063a9b-81f0-0310-95a4-ce76da25c4cd