feature: Command system almost-complete rework

[LichtHund]

Goals

• Allow nested commands.
• Arguments as sub commands.
• Custom annotations.

To achieve the goals I have decided to completely revamp quite a few of the systems.
This also came with an in my opinion improved syntax.

Syntax change

The previous syntax was made to avoid confusion and differentiate between method and class annotations.
Now there are no more difference on the annotations so the change makes sense.
I also find that default wasn't very clear what it meant, so I'm hoping the change also makes it more clear what each method does.

Old

@Command("foo")
public class MyCommand extends BaseCommand {
    
    @Default
    public void defaultSub(Sender sender) {} // /foo
    
    @SubCommand("bar")
    public void barSub(Sender sender) {} // /foo bar
}

New

@Command("foo")
public class MyCommand {
    
    @Command
    public void defaultSub(Sender sender) {} // /foo
    
    @Command("bar")
    public void barSub(Sender sender) {} // /foo bar
}

Nested Commands

The syntax for nested command will be the following:

@Command("foo")
public class MyCommand {
    
    @Command
    public void defaultSub(Sender sender) {} // /foo
    
    @Command("bar")
    public void barSub(Sender sender) {} // /foo bar

    @Command("nested")
    class Nested {
    
        @Command
        public void defaultSub(Sender sender) {} // /foo nested
    
        @Command("bar")
        public void barSub(Sender sender) {} // /foo nested bar
    }
}

Arguments as sub commands

The syntax for having arguments as sub commands.

@Command("foo")
public class MyCommand {

    @Command
    class Nested {

        private final String argument;
       
        public Nested(String argument) {
            this.argument = argument; // Passing the argument to be used in the sub command.
        }
   
        @Command
        public void defaultSub(Sender sender) {} // /foo <argument>
    
        @Command("bar")
        public void barSub(Sender sender) {} // /foo <argument> bar
    }
}

Custom annotations

Allow users to create their own annotations and behaviors to be added to the commands.
This is currently not prototyped. Once it is I'll edit the PR.

[bobooski]

This looks, respectfully, very sexy. I am a fan.

[Sparky983]

Amazing.