wgsl: Validate variable and value declaration parsing

[dj2]

• [ ] var a;
• [x] var a:i32; -- passes
• [ ] var i32:f32;
• [ ] var :i32;
• [ ] var a:invalid;
• [ ] var function> a:i32;
• [ ] var<> a:i32;
• [ ] var<function a:i32;
• [ ] var<invalid> a:i32;
• [ ] var<storage,> a:i32;
• [ ] var<storage,invalid> a:i32;
• [x] var<storage,read_write> a:i32; -- passes
• [ ] var<function,read_write> a:i32;
• [ ] var<function> :i32;
• [x] var a = 123; -- passes
• [ ] var a = invalid;
• [ ] var a:i32 = 1.0;
• [ ] let = 1;
• [ ] let a 1;
• [ ] let a =;
• [ ] let a;
• [ ] let a = 9ba;
• [ ] let a i32;
• [ ] let a := 123;
• [x] let a = 123; -- passes
• [x] let a:i32=123; --passes;
• [x] let a:i32;
• [ ] let a:invalid;
• [ ] let/* comment*/a/*comment*/:/* */i32/**/=/**/123;
• [ ] const a = 123; -- passes
• [x] const a:i32 = 1; -- passes
• [x] const a:i32 = 1.0;
• [ ] const :i32 = 1;
• [ ] const a;
• [x] const a:i32;
• [ ] const a i32 = 1;
• [ ] const a:=1;

Globals

• [ ] invalid attribute

• [x] valid attribute passes

• [x] multiple attributes passes

• [ ] override a;

• [x] override a = 1; -- passes

• [ ] override a:=1;

• [x] override a:i32=1; -- passes

• [ ] override a = invalid;

• [ ] override a:i32 1;

• [ ] override i32 = 2;

• [x] override a: i32 = 1.0f;

• [ ] override a i32 = 1;

• https://www.w3.org/TR/WGSL/#var-and-value-decl-grammar

[dj2]

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