[wgsl-in] Support const operations in const variables

[ashpil]

I'm not sure whether the official WGSL spec specifies anything about this, but I'm running into a pattern in my code where I'd like to do something like this:

const phi_sun: f32 = PI * 2.0;

However, the compiler doesn't like this:

error: expected separator, found '*'
   ┌─ wgsl:84:25
   │
84 │ const phi_sun: f32 = PI * 2.0;
   │                         ^ expected separator

error: aborting due to previous error

[kvark]

Unfortunately, the constant initializers in WGSL today can't do this - see const_expr grammar in the spec. We definitely do want to have this soon, just struggling with a good constexpr semantic.

[jimblandy]

WGSL issues:

• [wgsl] Support compile-time-constants (constexpr) gpuweb/gpuweb#1272
• Literal unification gpuweb/gpuweb#2200