patch 8.2.1641: Vim9: cannot use 0 or 1 where a bool is expected

Problem:    Vim9: cannot use 0 or 1 where a bool is expected.
Solution:   Allow using 0 and 1 for a bool type. (closes #6903)

src/structs.h

@@ -1373,6 +1373,7 @@ struct type_S {
 
 #define TTFLAG_VARARGS	1	    // func args ends with "..."
 #define TTFLAG_OPTARG	2	    // func arg type with "?"
+#define TTFLAG_BOOL_OK	4	    // can be converted to bool
 
 /*
  * Structure to hold an internal variable without a name.