[WebGPU] Document Read-only and read-write storage textures extension

[chrisdavidmills]

See https://developer.chrome.com/blog/new-in-webgpu-124#read-only_and_read-write_storage_textures for change description.

Acceptance criteria

• (If not already present) Add a Language extensions section in https://developer.mozilla.org/en-US/docs/Web/API/WGSLLanguageFeatures, and include a description of this one (readonly_and_readwrite_storage_textures). See https://gpuweb.github.io/gpuweb/wgsl/#language-extension for more info.
• Add information on the new access modes available to set in https://developer.mozilla.org/en-US/docs/Web/API/GPUDevice/createBindGroupLayout#access, and how to check whether they are available in the current implementation.
• Add BCD data points for this addition (both in WGSLLanguageFeatures and GPUDevice.createBindGroupLayout) — this shipped in Chrome 124.