Treat stomping an enemy as a jump

Previously, when the player stomped an enemy the enemy would set the
player's vertical velocity directly.

Instead, have the enemy call a new stomp() method on the player, which
internally triggers a jump having disarmed the double-jump. This makes a
difference in the following cases:

- If the player has a jump buffered, stomping on the enemy now consumes
  that jump.
- If the player is holding the jump key (because they buffered a jump)
  they can curtail the height of bouncing off the enemy.
- If double jump is enabled, it is now armed by stomping an enemy.

components/enemy/enemy.gd

@@ -71,7 +71,7 @@ func _on_gravity_changed(new_gravity):
 func _on_hitbox_body_entered(body):
 	if body.name == "Player":
 		if squashable and body.velocity.y > 0 and body.position.y < position.y:
-			body.velocity.y = body.jump_velocity
+			body.stomp()
 			queue_free()
 		elif player_loses_life:
 			Global.lives -= 1

scripts/player.gd

@@ -86,6 +86,22 @@ func _on_gravity_changed(new_gravity):
 	gravity = new_gravity
 
 
+func _jump():
+	velocity.y = jump_velocity
+	coyote_timer = 0
+	jump_buffer_timer = 0
+	if double_jump_armed:
+		double_jump_armed = false
+		_double_jump_particles.emitting = true
+	elif double_jump:
+		double_jump_armed = true
+
+
+func stomp():
+	double_jump_armed = false
+	_jump()
+
+
 func _physics_process(delta):
 	# Don't move if there are no lives left.
 	if Global.lives <= 0:
@@ -100,14 +116,7 @@ func _physics_process(delta):
 		jump_buffer_timer = (jump_buffer + delta)
 
 	if jump_buffer_timer > 0 and (double_jump_armed or coyote_timer > 0):
-		velocity.y = jump_velocity
-		coyote_timer = 0
-		jump_buffer_timer = 0
-		if double_jump_armed:
-			double_jump_armed = false
-			_double_jump_particles.emitting = true
-		elif double_jump:
-			double_jump_armed = true
+		_jump()
 
 	# Reduce velocity if the player lets go of the jump key before the apex.
 	# This allows controlling the height of the jump.