Twenty Sixteen: reimplement frontend JS w/o jQuery

[sgomes]

Reimplement Twenty Sixteen frontend JS without jQuery, and don't enqueue it as a result.

Trac ticket: https://core.trac.wordpress.org/ticket/55126

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[github-actions]

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[carolinan]

Hi, I did a quick manual test of the PR and when there is no menu setup, I am seeing the following:

functions.js?ver=20220209:41 Uncaught TypeError: Cannot read properties of null (reading 'querySelectorAll')
    at forEachNode (functions.js?ver=20220209:41:22)
    at initMainNavigation (functions.js?ver=20220209:48:3)
    at functions.js?ver=20220209:96:2
    at functions.js?ver=20220209:293:4

Once a menu is assigned to the primary menu location, the JS error is gone, but the keyboard navigation is not working: I can not open submenu items with the keyboard only.

[felixarntz]

This looks solid to me! The main open question which would allow simplifying this code quite a bit is whether we really still need to support IE11 here, given that core has officially dropped support a while ago.

[sgomes]

Thank you for taking a look, @carolinan and @felixarntz! The issues you mentioned should now be fixed.

As for IE11, I'll wait a bit longer for a consensus before making any changes on that front.

[sgomes]

Rebased on trunk.