This change adds the current tab index functionality to buttons and
implements a proof of concept for the new welcome page.

Primarily blocked on https://github.com/zed-industries/zed/pull/34804,
secondarily on https://github.com/zed-industries/zed/pull/35075 so we
can ensure navigation always works as intended.

Another thing to consider here is whether we want to assign the tab
order more implicitly / "automatically" based on the current layout
ordering. This would generally enable us to add a default order to
focusable elements if we want this. See [the
specification](https://html.spec.whatwg.org/multipage/interaction.html#flattened-tabindex-ordered-focus-navigation-scope)
on some more context on how the web usually handles this for focusable
elements.

Release Notes:

- N/A