WebGPURenderer: multiple canvas support

[aardgoose]

This introduces a CanvasRenderTarget() to represent a screen canvas, which can be used via renderer.setRenderTarget() for use with WebGPU, thus supporting multiple canvases.

The default screen canvas state in Renderer() is replaced by a CanvasRenderTarget(). - this may be too much code change?

[LeviPesin]

The API feels amazing!!

[sunag]

This PR sounds great to PR.
@mrdoob @Mugen87 Any thoughts on this approach?

const canvasRT = new CanvasRenderTarget( { domElement } );

renderer.setRenderTarget( canvasRT );

[Mugen87]

Is it necessary to introduce all the getActive*() methods? Could getPixelRatio() just work like so:

• Report the pixel ratio of the default canvas render target.
• If a custom canvas render target is set, return its values instead.

[aardgoose]

I considered that but was worried about altering the behaviour of those methods, happy to do that instead though, or add a method to getActiveCanvasTarget() for use within the Node classes.