Add MacOS trackpad zoom and offset gestures

[DSPerson]

I don‘ know this gesture is best solution, but it’s very useful for me

[imjp94]

@DSPerson Cool, I hope pan and magnify gesture will be supported by more OS in the future(https://github.com/godotengine/godot/issues/13139)

Just one thing before merging, can you make the scale variables as constants? Like MAGNIFY_GESTURE_SCALE and PAN_GESTURE_SCALE. It would be great if you can declare the constants after the signals from line 19

Thank you!