This PR implements an API inspired by https://github.com/mattjennings/excalibur-router/ with permission from Matt. The idea behind this refactoring is to add transitions between scenes and scene specific loaders! * Add or remove scenes by constructor * Add loaders by constructor * New `DefaultLoader` type that allows for easier custom loader creation * New `Transition` type for building custom transitions * New scene lifecycle to allow scene specific resource loading * `onTransition(direction: "in" | "out") {...}` * `onPreLoad(loader: DefaultLoader) {...}` * New async goto API that allows overriding loaders/transitions between scenes * Scenes now can have `async onInitialize` and `async onActivate`! * New scenes director API that allows upfront definition of scenes/transitions/loaders View [this file](https://github.com/excaliburjs/Excalibur/blob/feat/async-init-ex-router/sandbox/tests/router/index.ts) for a current example of what the API looks like Defining scenes upfront ```typescript const game = new ex.Engine({ scenes: { scene1: { scene: scene1, transitions: { out: new ex.FadeInOut({duration: 1000, direction: 'out', color: ex.Color.Black}), in: new ex.FadeInOut({duration: 1000, direction: 'in'}) } }, scene2: { scene: scene2, loader: ex.DefaultLoader, // Constructor only option! transitions: { out: new ex.FadeInOut({duration: 1000, direction: 'out'}), in: new ex.FadeInOut({duration: 1000, direction: 'in', color: ex.Color.Black }) } }, scene3: ex.Scene // Constructor only option! } }) game.start('scene1', { inTransition: new ex.FadeInOut({duration: 500, direction: 'in', color: ex.Color.ExcaliburBlue}) loader: boot, }); ```