Improve accessibility of media library vertical tabs

Problem/Motivation

This is a followup of #2988431: Assess and improve the accessibility of the Media Library field widget. The media library widget has several a11y issues. See #2988431-12: Assess and improve the accessibility of the Media Library field widget.

1. Vertical tabs menu is not presented as navigation
2. Clicking vertical tabs doesn’t shift the user to the new content, next element becomes the ‘Select media’ button

Proposed resolution

1. Add a nav element to the vertical tabs.
2. Don't move the focus after clicking a link in the vertical tabs. Leave focus on the link and announce the new content.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet