Docs: improve loader documentation

[KokoDoko]

Context

The loader documentation shows an example of creating your own loader screen, but this example is so barebones that it's not clear how to use it. The given example does not work as is. It's not clear what users have to add to get it working.

Proposal

Improve this example code so that it works out of the box.

import * as ex from 'excalibur';
export class MyLoader extends ex.DefaultLoader {
  override onUpdate(engine: Engine, elapsedMilliseconds: number): void {
  }
  override onDraw(ctx: CanvasRenderingContext2D) {
  }
  override async onUserAction(): Promise<void> {
  }
  override async onBeforeLoad(): Promise<void> {
  }
  override async onAfterLoad(): Promise<void> {
  }
}

[eonarheim]

@KokoDoko Totally agree!

[github-actions]

This issue hasn't had any recent activity lately and is being marked as stale automatically.