Accept only {f,i,u}32s when in strict WebGPU compliance

[ErichDonGubler]

Description

While writing #7016, I noticed that the diagnostic there stated that f64 and bool are accepted sampled types. That's great!…except that they shouldn't be allowed on strictly WebGPU-compliant environments. Section 6.5.2 of the WebGPU spec. states (in relation to sampled texture types):

The texture is parameterized by a sampled type and must be f32, i32, or u32.

Remedying this might require a new native-only feature, though one might already exist that I'm not aware of.

Repro steps

1. Run naga-cli on the following:

   @group(0) @binding(0) var texture: texture_2d<rgba32float>;

   Observe that it prints the following:

   Could not parse WGSL:
   error: unknown scalar type: `rgba32float`
     ┌─ ../in.wgsl:1:47
     │
   1 │ @group(0) @binding(0) var texture: texture_2d<rgba32float>;
     │                                               ^^^^^^^^^^^ unknown scalar type
     │
     = note: Valid scalar types are f32, f64, i32, u32, bool
   

Expected vs observed behavior

The set of "valid scalar types" (which could stand to be called "sampled types" instead) should f32, i32, and u32 when only the feature flags in wgpu::Features::all_webgpu_mask is set.