feat(a11y): expose the sidebar's tabs and the detail panel's tabs to screen readers

Each is a tab with its full title and selected state, and pressable by
assistive tech. Before, VoiceOver saw the window, its buttons and the
terminal, and nothing between them.

src/ui/tab_sidebar.rs

@@ -981,6 +981,11 @@ impl Tty7App {
 
                 let row = h_flex()
                     .id(("tab-row", i))
+                    // What a screen reader announces and presses: the full
+                    // title, not the elided one drawn.
+                    .role(gpui::Role::Tab)
+                    .aria_label(SharedString::from(title_text.to_string()))
+                    .aria_selected(is_active)
                     .group(SharedString::from(format!("tab-row-{i}")))
                     .cursor_pointer()
                     .on_drag(DragTab, {

src/ui/tab_strip.rs

@@ -1441,6 +1441,9 @@ impl Tty7App {
                 };
                 div()
                     .id(("right-panel-tab", tab as usize))
+                    .role(gpui::Role::Tab)
+                    .aria_label(t(label_key))
+                    .aria_selected(current)
                     // The press must not start a window drag from the title bar
                     // the tabs sit in.
                     .occlude()