wgsl: validate `global_directive`

[dj2]

• [ ] Multiple global directives are allowed
• [ ] Comment before global directive
• [ ] Whitespace before global directive
• [ ] All other declaration type fail
  • [ ] Type aliases
  • [ ] Module scoped variables
  • [ ] override declarations
  • [ ] creation-time constants
  • [ ] functions
  • [ ] struct declarations
  • [ ] Empty line with just ;
• [ ] staticAssert is not allowed before global directive.

References:

• https://www.w3.org/TR/WGSL/#directives

[dj2]

I believe these are all handled by the various specific directive validation tests. If there is something missing, please file a followup issue.