Add ISCPE constant for Support CPE

Added ISCPE constant for Support CPE flag.

Author: Trollhunters501

src/main/java/com/github/steveice10/mc/classic/protocol/ClassicConstants.java

@@ -21,6 +21,13 @@ public class ClassicConstants {
      */
     public static final String USERNAME_KEY = "username";
 
+    /**
+     * Flag key for Support CPE.
+     * <p>
+     * Available on both clients, set automatically based on provided isCPE.
+     */
+    public static final String ISCPE = "iscpe";
+
     /**
      * Flag key for a session's verification key.
      * <p>