Add screen reader announcement for opening/closing navigation pane (#20275)

## Summary of the Pull Request
Adds a manual event handler for `PaneOpened`/`PaneClosed` on the
navigation view in the settings UI. After catching the event, we raise a
UIA notification event to force the attached screen reader to read out
"Navigation pane opened/closed" appropriately. This experience aligns
with WinUI and the text is localized appropriately.

## Validation Steps Performed
✅ Narrator announces "Navigation pane opened/closed" appropriately
(aligns with WinUI 3 gallery experience)

## PR Checklist
Closes #19687 
📝 NOTE: the associated issue is currently affecting our accessibility
score