Tree: Implement WAI-ARIA treegrid keyboard navigation

Update Tree accessibility to follow the WAI-ARIA treegrid pattern:

Keyboard navigation changes:
- Right arrow: Expand collapsed row, or move to first cell if expanded
- Left arrow: Collapse expanded row, or move to row focus if on first cell
- Tab/Shift-Tab: Move between focusable elements (buttons, editable cells)
  in the current row, then exit tree when no more focusable elements
- Up/Down arrows: Navigate between rows (unchanged)

Accessibility improvements:
- Use ROLE_TREE_GRID for multi-column trees
- Use ROLE_ROW for tree items (instead of ROLE_TREE_ITEM)
- Use ROLE_CELL for individual column cells
- Use ROLE_COLUMN_HEADER for column titles
- Hide column headers from accessibility tree when not visible
- Parent buttons to row element (not cell) to prevent announcement clutter
- Only set expanded/collapsed state on items with children
- Set active_descendant on tree grid for screen reader focus tracking

Reference: https://www.w3.org/WAI/ARIA/apg/patterns/treegrid/

Author: ndarilek

doc/classes/DisplayServer.xml

@@ -2774,6 +2774,9 @@
 		<constant name="ROLE_TOOLTIP" value="45" enum="AccessibilityRole">
 			Tooltip element.
 		</constant>
+		<constant name="ROLE_TREE_GRID" value="46" enum="AccessibilityRole">
+			Tree grid element (tree with multiple columns).
+		</constant>
 		<constant name="POPUP_MENU" value="0" enum="AccessibilityPopupType">
 			Popup menu.
 		</constant>

drivers/accesskit/accessibility_driver_accesskit.cpp

@@ -1640,6 +1640,7 @@ AccessibilityDriverAccessKit::AccessibilityDriverAccessKit() {
 	role_map[DisplayServer::AccessibilityRole::ROLE_COLUMN_HEADER] = ACCESSKIT_ROLE_COLUMN_HEADER;
 	role_map[DisplayServer::AccessibilityRole::ROLE_TREE] = ACCESSKIT_ROLE_TREE;
 	role_map[DisplayServer::AccessibilityRole::ROLE_TREE_ITEM] = ACCESSKIT_ROLE_TREE_ITEM;
+	role_map[DisplayServer::AccessibilityRole::ROLE_TREE_GRID] = ACCESSKIT_ROLE_TREE_GRID;
 	role_map[DisplayServer::AccessibilityRole::ROLE_LIST] = ACCESSKIT_ROLE_LIST;
 	role_map[DisplayServer::AccessibilityRole::ROLE_LIST_ITEM] = ACCESSKIT_ROLE_LIST_ITEM;
 	role_map[DisplayServer::AccessibilityRole::ROLE_LIST_BOX] = ACCESSKIT_ROLE_LIST_BOX;