Switch to `WebGPURenderer`

More and more browsers are adding support for WebGPU, a newer interface than WebGL2 for controlling graphics hardware. Three.js has introduced `WebGPURenderer`, which is able to target either WebGPU or WebGL2 backends. Enabling Vol-E to run on top of WebGPU keeps us on top of the current web technology, and opens up the option to take advantage of WebGPU-only features like compute shaders in the future.

Three's `WebGPURenderer` also introduces a new node-based material system built on TSL, its new cross-platform shader language. Moving to this newer material system would enable us to modularize our update and/or render pipelines in creative and potentially productive ways. For example, we could separate data sampling (e.g. from a 3D texture or a 2D texture atlas) from render implementation (e.g. ray marching, path tracing), where currently each render mode is only compatible with exactly one data representation.