Feature that regenerates the repo nav menu to make it extensible

This is a solution to:

• #8867

I suggest we go ahead and add a new "invisible" feature that simply replaces the native "UnderlineNav" with a custom implementation. We will be able to use this base to restore the features:

• #9546
• #9549
• #9550
• #9548
• #9551

GitHub keeps all elements in the DOM, including those in the closed dropdown:

This means we should be able to loop through them and generate the new menu. The only part missing are the icons, which we'd have to manually match and load internally.

We can ship this extensible-nav feature and see if there's any pushback/issues we didn't anticipate. In a later release we can work towards actually adding bugs-tab and the rest.

As a first version, the nav should just overflow and introduce horizontal scrolling. This works well on mobile (pan to scroll) and should appear rarely on desktop (larger viewports)