extract menus into their own module

[scionaltera]

The menu code in the core module under com.agonyforge.mud.core.cli.menu.impl is a good candidate for refactoring into its own small module. It doesn't quite belong in the demo MUD because I don't want to make everybody implement their own menus from scratch if they don't want to. But it also doesn't quite belong in core because I'd expect most MUDs that get far enough along will want to customize how their menus look and not be forced to pull these default ones into their codebase. Having them in a separate module would walk that line pretty well: a sample implementation that can be added easily as a dependency for those who want it, and replaced for those who don't.