wgsl: Validate let declarations

[dj2]

• [x] let at module scope fails
• [x] let at function scope succeeds
• [x] let with shader-creation value succeeds
• [x] let with runtime value succeeds
• [x] let without a type infers the type
• [x] let with a type and non-matching RHS type fails
• [x] let with a type and matching RHS type succeeds
• [x] let without an initializer fails
• [x] let with invalid initializer fails
• [ ] let assigned a texture fails
• [ ] let assigned a sampler fails
• [x] let assigned a pointer passes ???
• [x] let assigned a pointer indirection passes let x = *y;
• [x] let assigned an atomic fails
• [x] let assigned a matrix passes
• [x] let assigned a vector passes
• [x] let assigned an array passes
• [x] let assigned runtime array fails?
• [ ] let with comments between components

[dj2]

Going to call this done. There appears to be sufficient coverage at this point. Followup issues can be filed if anything further is needed.

• https://github.com/gpuweb/cts/blob/main/src/webgpu/shader/validation/parse/var_and_let.spec.ts
• https://github.com/gpuweb/cts/blob/main/src/webgpu/shader/validation/decl/let.spec.ts