formatting applied to most of the source code to tidy things up, and misc refactors

src/main/java/com/gmail/nossr50/api/exceptions/InvalidPlayerException.java

@@ -1,6 +1,9 @@
 package com.gmail.nossr50.api.exceptions;
 
+import java.io.Serial;
+
 public class InvalidPlayerException extends RuntimeException {
+    @Serial
     private static final long serialVersionUID = 907213002618581385L;
 
     public InvalidPlayerException() {