Add Scoreboard API and Command. Adds BUKKIT-3776, BUKKIT-3834

The implementation is designed around having both a main scoreboard and
numberous plugin managed scoreboards that can be displayed to specific
players.

Plugin managed scoreboards are active so long as a reference is kept by a
plugin, or it has been registered as a player's active scoreboard. Objects
specific to a scoreboard remain active until unregistered (which remove a
reference to the owning scoreboard), but quickly fail if accessed
post-unregistration.

By: mbax <github@phozop.net>

paper-api/src/main/java/org/bukkit/command/SimpleCommandMap.java

@@ -57,6 +57,7 @@ public class SimpleCommandMap implements CommandMap {
         fallbackCommands.add(new EnchantCommand());
         fallbackCommands.add(new TestForCommand());
         fallbackCommands.add(new EffectCommand());
+        fallbackCommands.add(new ScoreboardCommand());
     }
 
     public SimpleCommandMap(final Server server) {