Add description to command nodes

[JesFot]

See #40 For an eventual help command purpose, a description of a node may be useful. In order to keep backward compatibility, I have duplicated some constructors.

[msftclas]

All CLA requirements met.

[liach]

Can't it become an info-displaying command that is bound to the node (like add this default command to all your tree nodes), so that for nodes that originally have no command like vanilla first literals (e.g. advancement datapack that reports unknown command on initial typing) will execute the dummy info command instead?

[JesFot]

Command or node description should not be interpreted as a command. To use your example: /advancement should only display that something is wrong with the command, not a description message about the command itself, /help advancement should do this.