Spec changes for atomic vec2u - #5610

On the fence about using vec2u in the core spec text. I think we generally expand it out to vec2<u32> (though it is likely used in examples).

I use it as the name of the enable !

On the fence about using vec2u in the core spec text. I think we generally expand it out to vec2<u32> (though it is likely used in examples).

I use it as the name of the enable !

I think it's ok there. More in the type definition is where I'm less sure.

Previews, as seen when this build job started (19bad54):
WebGPU _{webgpu.idl | Explainer | Correspondence Reference}
WGSL _{grammar.js | wgsl.lalr.txt}