Use native shader language of engine for node materials

To ensure BabylonJS will generate WGSL code from node materials when using WebGPU, we need to set the following parameter:

https://doc.babylonjs.com/typedoc/classes/BABYLON.NodeMaterial#usenativeshaderlanguageofengine

To complete this issue, set this static member to `true` after creating the engine in `cosmosjourneyer.ts` and `playground.ts`