Add a Command parameter type for custom commands

Custom commands currently support String, Enum, PlayerSelector, etc. but there is no parameter type for a nested command. Vanilla /execute ... run <command> has this, with full autocomplete and syntax validation, but custom commands can't reuse it.

Right now the only workaround is taking a String and calling runCommand, which has no autocomplete, needs manual quote escaping, and gives no syntax validation before execution.

Please add a command parameter type that exposes the same command parser as /execute run. Use cases: custom commands that trigger an action after a delay, per-player hooks like /mycmd @a "title @s title Hi", and wrapper commands that extend /execute.