wgsl: numeric params of attributes should be creation-time expressions not literals

[dneto0]

Numeric parameters to attributes are specified as either positive i32 literal or non-negative i32 literal.

Instead of saying i32 literal, they should be i32 creation-time expression.

This would allow things like:

const block_size = 16;
@compute @workgroup_size(block_size)
fn foo(...) { ... }

[dneto0]

I think this was intended but didn't make it into the constant-time expr spec update.

[alan-baker]

Dupe of #2790