[wgsl-in] Handle repeated or missing @workgroup_size

[fornwall]

Fixes #2271 and also fixes #2434.

[fornwall]

Change for missing @workgroup_size (#2271):

With the below shader:

@compute
fn main() {}

Before:

After:

[fornwall]

Change for repeated @workgroup_size (#2434):

For the below shader:

@compute @workgroup_size(1) @workgroup_size(1)
fn main() {}

Before:

After:

[teoxoy]

There is a bigger question if we should make it optional at the IR level but this looks good for now.

Thanks for the recent contributions! :)