WGSL: Allow global let-declarations initialized with constant expressions #1911

Current WGSL SPEC only allow global let-declarations initialized with const_expr, and const_expr only accepts const_literal.

It would be convenient for developers to support initializing global let-declarations with "constant expressions" not just "constant numbers". For example, the following WGSL expressions will become valid after WGSL supports initializing global let-declarations with "constant expressions":

let size: u32 = 4u * 5u;

let a: u32 = 4u;
let b: u32 = a;