Rename repo to lowercase `webxr-webgpu-binding`

[tidoust]

The draft spec advertizes its URL as https://immersive-web.github.io/webxr-webgpu-binding/ which yields a 404 because GitHub Pages are case-sensitive and current repository name is WebXR-WebGPU-Binding. Since both the WebXR and WebGPU specs use lower case shortnames, it seems good to have everything in lower case. Could the repository be renamed to webxr-webgpu-binding to fix the broken link?

[dmitshur]

Issue immersive-web/webxr-samples#208 is somewhat related to this.

[himorin]

/cc @AdaRoseCannon @toji

[toji]

I have no concerns about renaming.