Bundled Themes: Twenty Twenty anchor links don't work in mobile menu.

Modifies the mobile modal menu javascript, so that anchor links will close the modal and scroll to the anchor within the page.

Props Giorgio25b, suzylah, yuhin, samful, bdcstr.
Fixes #48916.
Built from https://develop.svn.wordpress.org/trunk@47784


git-svn-id: https://core.svn.wordpress.org/trunk@47560 1a063a9b-81f0-0310-95a4-ce76da25c4cd