Unify the event style of engine

[shrinktofit]

Re: https://discord.gg/cocos-simplify-game-development-634690911896469525

[shrinktofit]

In discussion with @PPpro , we prefer the on() style registering:

class A extends EventTarget {
  on(type: "event-a", (callback_signature_of_event_a) => void): void;
  on(type: "event-b", (callback_signature_of_event_b) => void): void;
  on(type: string, callback: (...args: unknown[]) => void) {
    super.on(type, callback); // Forward to `super.on`
  }
}

[pandamicro]

@lealzhan please check the event callbacks in physics module @holycanvas please check the event callbacks in 2D & UI module

[PPpro]

some developers also have some feedback for event registration on Spine module @zhakesi https://forum.cocos.org/t/topic/148276/64?u=_pp