[PoC] Adapt example_glfw_wgpu to wgpu-native

[eliemichel]

Context: This PoC comes as a complement to a remark in #7435.

Problems:

• The current version of example_glfw_wgpu is tied to Dawn implementation for multiple reasons (see this comment).
• It is hard at this stage of WebGPU to have a code that is truly implem-agnostic.

This PR shows an alternative version of example_glfw_wgpu that uses wgpu-native, and which is in general closer to what an implem-agnostic version would look like. It uses precompiled binaries, which makes building the example for quick testing much faster than building Dawn.

Receipe to make this implem-agnostic:

1. Wait until the next release of emscripten so that they get the new Surface Configuration API (already available in Dawn and wgpu-native), which replaces the deprecated SwapChain API (dropped from the standard ~8 months ago and no longer provided by wgpu-native)
2. Replace FetchWgpuNative.cmake by e.g., the webgpu.cmake file from my WebGPU-distribution repo (I did not want to impose this by default since I'm clearly biased in favor of my tool here -- my ultimate goal is that one day this whole distribution thing will no longer be needed).
3. Tell people to make sure that if they provide a WebGPU implem themselves they should take care of defining either WEBGPU_BACKEND_WGPU or WEBGPU_BACKEND_DAWN themselves as well.

Due to all these reason I suggest not to merge this right away, but keeping the PR as a reference may be useful for the very near future.