Reduce `min()`, `max()` and `clamp()` with number arguments (#1131)

- Add support for `Calc::Number` in reduce_args to simplify `min()/max()/clamp()`
  with plain number arguments (e.g., `min(1, 2, 3) => min(1, 2)`)
- Extend clamp() parsing to handle Number comparisons (e.g.,
  `clamp(0, 255, 300) => 255`)

Fixed: #1044, #1129