Allow choosing the underlying WebGPU library via command-line flags

[hujiajie]

This is the WebGPU counterpart of #101, so the following modes should be supported by Aquarium:

• Do not link with system WebGPU at build time, except in the WASM build
• Dynamically load system WebGPU when Aquarium is asked to use the WebGPU backend
• Dynamically load Dawn when Aquarium is asked to use the Dawn backend

Dynamically loading system WebGPU is not a quite useful mode at the moment, but it leaves the door open in case some day Dawn or wgpu-native can be installed system-wide.

One challenge is that dynamic loading implies using the C binding, while Aquarium is built on top of the C++ APIs. It should also be noted that the C++ wrapper is not shipped in Emscripten.