Toggle fullscreen on super/ctrl+return, only macOS for now

This fixes or at least is the first step towards #171:

- it adds `cmd/super + return` as the default keybinding to toggle
  fullscreen for currently focused window.
- it adds a keybinding handler to the embedded apprt and then changes
  the macOS app to handle the keybinding by toggling currently focused
  window.

src/input/Binding.zig

@@ -207,6 +207,9 @@ pub const Action = union(enum) {
     /// Close the window, regardless of how many tabs or splits there may be.
     close_window: void,
 
+    /// Toggle fullscreen mode of window.
+    toggle_fullscreen: void,
+
     /// Quit ghostty
     quit: void,