Support emdawnwebgpu as a webgpu backend

[alexreinking]

Since the webgpu backend was developed, Google has released emdawnwebgpu

Emdawnwebgpu is Dawn‘s implementation of webgpu.h for Emscripten (on top of the WebGPU JS API). It is a fork of Emscripten’s original USE_WEBGPU bindings, and while it is maintained in Dawn, it works in any browser supporting WebGPU (modulo individual feature support).

Emdawnwebgpu provides everything necessary to use <webgpu/webgpu.h> and the Dawn-style <webgpu/webgpu_cpp.h> with Emscripten.

We should support this backend.