SPIGOT-6899: Add Player#getPreviousGameMode

By: Doc <nachito94@msn.com>

paper-api/src/main/java/org/bukkit/entity/Player.java

@@ -570,6 +570,14 @@ public interface Player extends HumanEntity, Conversable, OfflinePlayer, PluginM
     @Deprecated
     public void updateInventory();
 
+    /**
+     * Gets this player's previous {@link GameMode}
+     *
+     * @return Previous game mode or null
+     */
+    @Nullable
+    public GameMode getPreviousGameMode();
+
     /**
      * Sets the current time on the player's client. When relative is true the
      * player's time will be kept synchronized to its world time with the