feat: add MenuItem.userAccelerator property (#26682)

* feat: add MenuItem.userAccelerator property

This allows folks to read the user-assigned accelerator that macOS users can provide in system preferences. Useful for showing in-app shortcut help dialogs, you need to know if the accelerator you provided is not being used in favor of a user assigned one.

* chore: update syntax

* chore: add safety check for command index being -1

shell/browser/ui/cocoa/electron_menu_controller.h

@@ -52,6 +52,10 @@ class ElectronMenuModel;
 // default initializer was used, then this will create the menu on first call.
 - (NSMenu*)menu;
 
+- (base::scoped_nsobject<NSMenuItem>)
+    makeMenuItemForIndex:(NSInteger)index
+               fromModel:(electron::ElectronMenuModel*)model;
+
 // Whether the menu is currently open.
 - (BOOL)isMenuOpen;