patch 9.0.1625: "super" is not considered a reserved name

Problem:    "super" is not considered a reserved name.
Solution:   Add "super" to the list of reserved names. (closes #12515)

src/vim9script.c

@@ -1122,6 +1122,7 @@ static char *reserved[] = {
     "null_string",
     "null_channel",
     "null_job",
+    "super",
     "this",
     NULL
 };