[wgsl-in] Refuse to apply postfix expressions to WGSL pointers.

Fixes #1356.

Output for the test case in that issue is now:

    error: the value indexed by a `[]` subscripting expression must not be a pointer
      ┌─ wgsl:5:14
      │
    5 │     let a = *pv[3]; // Problematic line
      │              ^^ expression is a pointer

    Could not parse WGSL