Add a 'not all' or 'nand' condition to the execute command

We need a way to check for multiple blocks but allow a margin of error. Implementing a 'not all' condition would be a game-changer. For example, if I have 3 target positions, the command should only fail if all 3 are present. If only 1 or 2 are present, it should still execute. The current unless is too strict (fails on 1) and if is too restrictive (requires all 3).

Exemple: /execute nand block -41 -57 -16 anvil nand block -41 -57 -17 anvil

If you have one anvil, execute the action there, but if you have both, do not execute it