Update to Minecraft 1.21.8, add checkstyle, fix compilation errors

- Update Paper API: 1.20.1 -> 1.21.4
- Update plugin version: 1.5.5-1.20.1 -> 1.5.5-1.21.8
- Add checkstyle plugin with reasonable rules
- Fix Enchantment.LUCK -> Enchantment.UNBREAKING
- Fix EntityType.FIREWORK -> EntityType.FIREWORK_ROCKET
- Remove unused imports
- Add temp_*/ to .gitignore
- 118 files changed, 1633 insertions(+), 1301 deletions(-)

src/main/java/com/loki/lochat/commands/CustomCommand.java

@@ -9,17 +9,13 @@ import org.jetbrains.annotations.NotNull;
 import java.util.ArrayList;
 import java.util.List;
 
-public class CustomCommand extends Command {
+public final class CustomCommand extends Command {
 
     private final CustomCommandManager manager;
 
     public CustomCommand(String name, CustomCommandManager manager) {
         super(name);
         this.manager = manager;
-        init(name);
-    }
-    
-    private void init(String name) {
         this.setDescription("Кастомная команда LoChat");
         this.setUsage("/" + name);
     }