wgsl: validate parsing of type aliases

[dj2]

• [ ] type alias to an existing type alias passes
  • [ ] Existing alias before new alias
  • [ ] Existing alias after new alias
• [ ] type alias to a non-existing type fails
• [ ] type a = struct { b: i32} fails
• [x] type alias to each existing type passes
  • [x] bool
  • [x] f32
  • [x] f16
  • [x] f32
  • [x] u32
  • [x] vecN<f32>
  • [x] matCxR<f32>
  • [x] previously declared structure
  • [x] structure declared after the alias
  • [x] ptr<function, i32, read>
  • [x] array<f32, 5>
  • [x] array<i32>
  • [x] atomic<i32>
  • [x] {each of the texture types}
  • [x] sampler
  • [ ] sampler_comparison
• [ ] type alias with a name matching an existing type fails
• [ ] type alias missing = fails
• [ ] type alias missing type fails
• [ ] type alias missing name fails
• [ ] valid type alias inside a function fails
• [ ] type alias to invalid type fails type a = ptr;

4.7. Type Aliases

[dj2]

https://github.com/gpuweb/cts/blob/main/src/webgpu/shader/validation/types/alias.spec.ts