Correctness and diagnostic fixes

[SparkyPotato]

• fixes [wgsl-in] Parser::block does not check for leading { #2216
• fixes [wgsl-in] panic on malformed input #2209
• fixes [wgsl-in] Invalid assignment diagnostic doesn't extend to derived expressions #2169

Also checks for the ; after break, continue, and discard, as per the spec.

The diagnostic for #2209 is a bit weird:

error: type `array<f32, 0f>` is not constructible
  ┌─ wgsl:2:6
  │
2 │     -array<f32, 0f>() = v;
  │      ^^^^^^^^^^^^^^ type is not constructible

But it does point the user to where the problem is and doesn't crash.

The invalid assignment diagnostic is also improved overall:

error: invalid left-hand side of assignment
  ┌─ wgsl:2:9
  │
2 │     let a = array(1, 2);
  │         ^ this is an immutable binding
3 │     a[0] = 2;
  │     ^^^^ cannot assign to this expression
  │
  = note: consider declaring 'a' with `var` instead of `let`

[teoxoy]

This looks great! Just need to resolve the conflict.