Command handler register overhaul

[codemasher]

As per the conversation in #1413.

[codemasher]

@valzargaming re this comment:
I see why this might be a breaking change for some, but I think the main issue is that there is no clear behaviour to this method (not to mention the lack of documentation), so people might be using it in unexpected ways. I guess there's a wider (definitely breaking) overhaul necessary, also in order to properly accommodate subcommand groups.

Right now, there seems an issue with registering a command with subcommands in one go, like so:

$discord->listenCommand(['command', 'subcommand1', 'subcommand2', /* ... */], execute(...), autocomplete(...));

Registering a single command and adding subcommands via the method on the RegisteredCommand instance works as expected:

$registeredCommand = $discord->listenCommand('command', execute(...), autocomplete(...));

foreach(['subcommand1', 'subcommand2', /* ... */] as $subcommand){
    $registeredCommand->addSubCommand($subcommand, execute(...), autocomplete(...));
}

Then apparently there's this case too, as mentioned in the other issue:

$discord->listenCommand(['command', 'subcommand1'], execute(...), autocomplete(...));
$discord->listenCommand(['command', 'subcommand2'], execute(...), autocomplete(...));
/* ... */

Further I can see it being used somehow like this:

$discord->listenCommand(['command', ['subcommandgroup1', 'subcommand1'], ['subcommandgroup2', 'subcommand1'], /* ... */], execute(...), autocomplete(...));

All in all I think that's too much complexity and error prone behaviour to handle (I don't even know yet where and how subcommand groups are handled). The method should offer stricter parameters, and/or additional methods that properly handle a single task in a concise way should be added.

Discord::listenCommand(
    string $name, 
    ?string $subcommandGroup = null, 
    array $subcommands = [], 
    ?callable $callback = null, 
    ?callable $autocomplete_callback = null.
):RegisteredCommand

This way it should be possible to call the method repeatedly without throwing and handle the internals without confusion:

$discord->listenCommand(name: 'command', callback: execute(...), autocomplete_callback: autocomplete(...));
$discord->listenCommand('command', 'subcommandgroup1', ['subcommand1', /* ... */], execute(...), autocomplete(...));
$discord->listenCommand('command', 'subcommandgroup2', ['subcommand1', /* ... */], execute(...), autocomplete(...));

[valzargaming]

@Log1x, whenever you get the chance, what are your thoughts on this? I know Laracord abstracts a lot of this logic, and I think changing things as they are would break the existing framework.