wgsl: Validate `override` declarations

[dj2]

• [x] Declaration at module scope succeeds
• [x] Declaration at other scopes fail
• [x] Declaration with a scalar type and initializer succeeds
  • [x] bool
  • [x] i32
  • [x] u32
  • [x] f32
  • [x] f16
• [x] Declaration with a scalar type and no initializer succeeds
• [x] Declaration with no type and a scalar initializer succeeds
• [x] Declaration without a type and without an initializer fails
• [ ] Declaration with a non-scalar initializer fails
• [x] Declaration with a non-scalar type fails
• [x] Declaration with the type and initializer are both scalar but don't match fails
• [x] Declaration with only creation-time expressions passes
• [x] Declaration with overridable constants as initializer passes
• [ ] Declaration with out-of-order overridable constants in initializer passes
• [x] Declaration with creation-time constants passes
• [ ] Declaration with creation-time functions passes
• [ ] Declaration w/ non-creation-time function fails
• [x] Declaration with an invalid @id fails

[dj2]

• [x] override width = 1; succeeds

[dj2]

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