QOL: Add some java commands

Added many commands from Minecraft Java edition:

    setblock
    fill
    seed
    xp
    gamemode spectator
    summon
    effect
    difficulty
    spawnpoint
    setworldspawn
    kick
    enchant

All of the logic is contained inside of PlayerConnection.cpp along with the previous command handler, i also changed MobEffect.h to add a new helper for potion names -> ids.
And for spectator, a bunch of small changes inside server/client related file, along with the removal of the weird /spectator command which was added previously. I think it can still be refined because im not sure the invisibility thing is working.

Minecraft.Client/ServerPlayer.cpp

@@ -1987,8 +1987,11 @@ void ServerPlayer::restoreFrom(shared_ptr<Player> oldPlayer, bool restoreAll)
 
 void ServerPlayer::onEffectAdded(MobEffectInstance *effect)
 {
-	Player::onEffectAdded(effect);
-	connection->send(std::make_shared<UpdateMobEffectPacket>(entityId, effect));
+    if (this->connection != nullptr)
+    {
+       	Player::onEffectAdded(effect);
+        connection->send(std::make_shared<UpdateMobEffectPacket>(entityId, effect));
+    }
 }
 
 
@@ -2001,8 +2004,11 @@ void ServerPlayer::onEffectUpdated(MobEffectInstance *effect, bool doRefreshAttr
 
 void ServerPlayer::onEffectRemoved(MobEffectInstance *effect)
 {
-	Player::onEffectRemoved(effect);
-	connection->send(std::make_shared<RemoveMobEffectPacket>(entityId, effect));
+    if (this->connection != nullptr)
+    {
+        Player::onEffectRemoved(effect);
+        this->connection->send(std::make_shared<RemoveMobEffectPacket>(this->entityId, effect));
+    }
 }
 
 void ServerPlayer::teleportTo(double x, double y, double z)
@@ -2151,4 +2157,4 @@ bool ServerPlayer::isSpectator()
         return false;
     }
     return gameMode->getGameType() == GameType::SPECTATOR;
-}
+}