refactor: Make second argument in ADD / MUL variadic (#1312)

This commit changes the function signature of the ADD and MUL functions, i.e.
```diff
FUNCTION ADD < T1: ANY, T2: ANY >: T1
    VAR_INPUT
        IN1: T1;
-       IN1: T2;
+       IN2: T2...;
    END_VAR
END_FUNCTION
```

(This should not change the behavior when calling some specific `ADD__...` function either implicitly or
explicitly because the validation should catch such cases)

Author: volsa

libs/stdlib/iec61131-st/arithmetic_functions.st

@@ -12,6 +12,32 @@ VAR_GLOBAL
     E_LREAL : LREAL;
 END_VAR
 
+(********************
+*
+* This operator returns the value of adding up the operands.
+* It overloads the variadic builtin implementation of ADD, which is implemented for ANY_NUM
+*
+*********************)
+FUNCTION ADD < T1: ANY, T2: ANY >: T1
+VAR_INPUT
+    IN1: T1;
+    IN2: T2...;
+END_VAR
+END_FUNCTION
+
+(********************
+*
+* This operator produces the multiplication of the operands.
+* It overloads the variadic builtin implementation of MUL, which is implemented for ANY_NUM
+*
+*********************)
+FUNCTION MUL < T1: ANY, T2: ANY >: T1
+VAR_INPUT
+    IN1: T1;
+    IN2: T2...;
+END_VAR
+END_FUNCTION
+
 (* Calculates the square root of a given value *)
 {external}
 FUNCTION SQRT<T : ANY_REAL> : T

libs/stdlib/iec61131-st/date_time_numeric_functions.st

@@ -1,16 +1,3 @@
-(********************
-*
-* This operator returns the value of adding up the operands.
-* It overloads the variadic builtin implementation of ADD, which is impemented for ANY_NUM
-*
-*********************)
-FUNCTION ADD < T1: ANY, T2: ANY >: T1
-VAR_INPUT
-    IN1: T1;
-    IN2: T2;
-END_VAR
-END_FUNCTION
-
 (* Specialized implementation of ADD for TIME *)
 FUNCTION ADD__TIME__TIME: TIME
 VAR_INPUT
@@ -340,19 +327,6 @@ END_VAR
     SUB_LDT_LDT := SUB_DT_DT(IN1, IN2);
 END_FUNCTION
 
-(********************
-*
-* This operator produces the multiplication of the operands.
-* It overloads the variadic builtin implementation of MUL, which is impemented for ANY_NUM
-*
-*********************)
-FUNCTION MUL < T1: ANY, T2: ANY >: T1
-VAR_INPUT
-    IN1: T1;
-    IN2: T2;
-END_VAR
-END_FUNCTION
-
 (********************
 *
 * This operator produces the multiplication of TIME and ANY_NUM.