[WebGPU] Document pointer_composite_access extension

[chrisdavidmills]

See https://developer.chrome.com/blog/new-in-webgpu-123#syntax_sugar_for_dereferencing_composites_in_wgsl for feature 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 (pointer_composite_access). See https://gpuweb.github.io/gpuweb/wgsl/#language-extension for more info.
• Add BCD data points for this addition (in WGSLLanguageFeatures) — this shipped in Chrome 123.