Support `@must_use` function attribute in WGSL

[ErichDonGubler]

We do not currently support the @must_use function attribute, despite being specified in the current v1 spec. draft. This was introduced in early 2023; see gpuweb/gpuweb#3862, gpuweb/gpuweb#3819.

Repro steps

^{Ideally, a runnable example we can check out.}

1. With this in.wgsl input file:

   @must_use 
   fn helper() -> u32 {
   	return 2;
   }
   
   @compute @workgroup_size(1)
   fn comp_main() {
   	let asdf = helper();
   }
   

   …run this on latest trunk (ATOW, d239361c1f5d5cfb8cee73382b1e8bc8476e3b29):

   $ cargo run -p naga-cli --all-features -- in.wgsl
   

Expected vs observed behavior

^{Clearly describe what you get, and how it goes across your expectations.}

This should compile without error. The asdf binding is unused, but that should, at most, be a warning. Instead, the current output of repro steps is:

Could not parse WGSL:
error: unknown attribute: 'must_use'
  ┌─ in.wgsl:1:2
  │
1 │ @must_use
  │  ^^^^^^^^ unknown attribute

[ErichDonGubler]

Firefox bug: bug 1878314

[jimblandy]

This is a higher priority than it might appear, because it prevents some popular WebGPU demos from running.