patch 8.1.2341: not so easy to interrupt a script programatically

Problem:    Not so easy to interrupt a script programatically.
Solution:   Add the interrupt() function. (Yasuhiro Matsumoto, closes #2834)

src/ex_eval.c

@@ -85,6 +85,7 @@ static int cause_abort = FALSE;
  * until the throw point for error messages has been reached.  That is, during
  * cancellation of an expression evaluation after an aborting function call or
  * due to a parsing error, aborting() always returns the same value.
+ * "got_int" is also set by calling interrupt().
  */
     int
 aborting(void)