PCSS support for WebGPURenderer.

[supermoos]

Would be amazing to have PCSS ported for use with the WebGPURenderer.

Any pointers of previous drei shaders or similar that may already have been ported is much appreciated, then I can perhaps take a stab at it myself.

Cross linked issue: https://github.com/pmndrs/drei-vanilla/issues/52

[CodyJasonBennett]

WebGPURenderer is still heavily under development, and there's no user-facing API at this time.

It may be at least a year until this is even possible as experimental, but that's not befitting of a library under SemVer.