ui: Wire up tab indices within buttons (#35368)

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

crates/ui/src/components/button/icon_button.rs

@@ -164,6 +164,11 @@ impl ButtonCommon for IconButton {
         self
     }
 
+    fn tab_index(mut self, tab_index: impl Into<isize>) -> Self {
+        self.base = self.base.tab_index(tab_index);
+        self
+    }
+
     fn layer(mut self, elevation: ElevationIndex) -> Self {
         self.base = self.base.layer(elevation);
         self