Support command data graph (tab completion for 1.13+)

[Vitaly]

https://wiki.vg/Command_Data

on login server seems to send declare_commands (0x10) packet that looks like this:

packet data example

{
    nodes: [
        {
            "flags": {
                "unused": 0,
                "has_custom_suggestions": 0,
                "has_redirect_node": 0,
                "has_command": 0,
                "command_node_type": 0
            },
            "children": [
                1,
                2,
                3,
                4,
                5,
                6,
                7,
                8,
                9
            ]
        },
        {
            "flags": {
                "unused": 0,
                "has_custom_suggestions": 0,
                "has_redirect_node": 0,
                "has_command": 1,
                "command_node_type": 1
            },
            "children": [
                11
            ],
            "extraNodeData": {
                "name": "help"
            }
        }
    ],
    rootIndex: 0
}

don't think it would hard to implement, just posting it here for the record.