[Enhancement] Execute MCDC commands in-game

[Xujiayao]

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Related Issues

Fix #45

Description

This PR adds the ability to execute MCDC commands in-game. As of the time this PR was merged, the available commands are as follows:

Command	Description
/mcdc info	Query server running status
/mcdc help	Get a list of available commands
/mcdc update	Check for update
/mcdc stats	Query the scoreboard of a statistic
/mcdc reload	Reload MCDiscordChat config file (admin only)
/mcdc console	Execute a command in the server console (admin only)
/mcdc log	Get the specified server log (admin only)
/mcdc stop	Stop the server (admin only)

Strikethrough commands are not implemented because they are redundant or impossible to implement in-game.

Admin-only commands require a level 4 operator at minimum, i.e. players with OP permission level 4 or the server console.