Add scripting to stage/rooms

[zackthehuman]

Stages should have a scripting layer that allows some aspects of the stage to be controlled by the script. This would allow for things like in-stage cinematics (ala Breakman appearing in Gemini man's stage -- KAPOW!).

Scripts should be optional, and there should be a base class that a stage script must extend in order for the game to call in to it correctly.

class StageBase {
    constructor() { }

    onUpdate(dt) { }

    onLeaveRoom(roomId) { }

    onEnterRoom(roomId) { }

    onStartStage() { }

    onStartRound() { }

    onEndRound() { }

    onStartBossBattle() { }

    endBossBattle(alreadyDefeated) { }
}