a11y: Landmarks and menu improvements (#60397)

Fixes menu a11y, and adds landmarks with `F6`-navigation.

Also fixes a GPUI bug, and adds debug actions for dumping a11y tree
info.

Since `F6` was already in use by the pause debugger keybind, also
tightens up the debugger keybind context so they require an active
debugger session. When there is one active, `F6` stays as pause
debugger. `ctrl-F6` always works to go to the next landmark.

Also adds an "accessible mode" setting. Currently, this only controls
whether we show all menus all the time, but I suspect it will expand
significantly in the future.

Also adds `.aria_keyshortcuts()` API, but it's not wired up within
accesskit adapters, so is not yet reported to screen readers.

---

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/encoding_selector/src/active_buffer_encoding.rs

@@ -88,6 +88,7 @@ impl Render for ActiveBufferEncoding {
         div().child(
             Button::new("change-encoding", text)
                 .label_size(LabelSize::Small)
+                .tab_index(0isize)
                 .on_click(cx.listener(move |this, _, window, cx| {
                     if disabled {
                         return;