feat: Scene input API

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

==================

This PR enables scene specific input APIs that only fire handlers when the scene is active! This is very useful if you have a lot of handlers to manage and don't want to keep track of engine level subscriptions.

class SceneWithInput extends ex.Scene {
  onInitialize(engine: ex.Engine<any>): void {
    this.input.pointers.on('down', () => {
      console.log('pointer down from scene1');
    });
  }
}
class OtherSceneWithInput extends ex.Scene {
  onInitialize(engine: ex.Engine<any>): void {
    this.input.pointers.on('down', () => {
      console.log('pointer down from scene2');
    });
  }
}

scene-input.mp4