Add CommandMap#unregister method

[A248]

Is your feature request related to a problem?

With the advent of Paper plugins, calling into the CommandMap for registration is now recommended over the plugin.yml's commands section. The CommandMap#register method exists; however, there is no #unregister method. Yet plugins would benefit from de-registering; for example, if the plugin implements configurable aliases, or if it is a dedicated alias plugin outright.

Describe the solution you'd like.

Add a CommandMap#unregister method to remove a command entirely.

Describe alternatives you've considered.

Continue to instruct developers to mutate the known commands map returned from CommandMap#getKnownCommands. This solution is more error-prone and less discoverable. It is easy to call, for example, commandMap.getKnownCommands().values().remove(myCommand) without realizing the provided command may be registered under multiple aliases.

Also, getKnownCommands() is not documented as mutable. In fact, command unregistration via the CommandMap is not documented at all. The reason for this is that programmatic registration remained a rare use-case hardly possible on older API versions. For example, Server#getCommandMap was added by Paper and didn't exist in Spigot for a while (maybe it still doesn't -- I don't know). Hence, some plugins reflect into SimplePluginManager to get the command map, with an extra reflection step to access the known commands field.

Other

No response