Modify setBalance to return updated balance

Changed the return type of the setBalance method in the Account class from void to BigDecimal. This update allows the method to return the updated balance after setting it.

api/src/main/java/net/thenextlvl/economist/api/Account.java

@@ -63,6 +63,7 @@ default int compareTo(Account account) {
      * Sets the balance of the account to the specified value.
      *
      * @param balance the new balance of the account
+     * @return the updated balance of the account
      */
-    void setBalance(Number balance);
+    BigDecimal setBalance(Number balance);
 }