wgsl: validate scoping rules

[dj2]

• [ ] Access variable not in-scope is invalid
• [ ] Shadow a module scoped variable
  • [x] Module-scoped variable declared before function
  • [ ] Module-scoped variable declared after function
• [x] Shadow a function name inside the function allowed
• [x] Shadow a function name as a parameter name is allowed
• [x] Shadow variable in inner scope allowed
• [ ] Shadow variable in same scope denied.
• [x] Shadow of builtin function
  • [x] #3705
  • [x] Inside a function is allowed
• [x] Shadow a parameter name inside a block
• [x] Shadow of a for scoped variable
• [ ] Shadow of a type alias name
• [ ] Shadow of a struct name
• [ ] Use before definition
  • [ ] type alias
  • [ ] module scoped variable
  • [ ] struct
  • [x] call a function
• [ ] Validate that the following are rejected when a shadow is in scope
  • [ ] a function name, and then trying to call the function
  • [ ] a type alias, and then trying to use as a type
  • [ ] a struct name, and then trying to use the struct
  • [ ] a built-in function name, and then trying to call the function
• [ ] Validate that a non-module scoped identifier can only be used after declaration

See example in https://www.w3.org/TR/WGSL/#declaration-and-scope for possible others.

[dj2]

• Builtin shadowing tests: https://github.com/gpuweb/cts/blob/main/src/webgpu/shader/validation/parse/shadow_builtins.spec.ts
• Other shadowing tests: https://github.com/gpuweb/cts/blob/main/src/webgpu/shader/execution/shadow.spec.ts