Add ^switched-emitter, test in level 4

modules/game/level.scm

@@ -83,6 +83,7 @@
                                   (target-y (bytevector-u8-ref objects (+ i 4))))
                               (spawn ^electron-warp x y target-x target-y)))
                         (15 (spawn ^or-gate x y))
+                        (16 (spawn ^switched-emitter x y 4))
                         (id (error "invalid level object" id))))
                  (i* (+ i (match id
                             ;; floor-switch