patch 8.2.1308: Vim9: accidentally using "x" causes Vim to exit

Problem:    Vim9: accidentally using "x" causes Vim to exit.
Solution:   Disallow using ":x" or "xit" in Vim9 script. (closes #6399)

src/vim9script.c

@@ -58,13 +58,30 @@ ex_vim9script(exarg_T *eap)
     }
 }
 
+/*
+ * When in Vim9 script give an error and return FAIL.
+ */
+    int
+not_in_vim9(exarg_T *eap)
+{
+    switch (eap->cmdidx)
+    {
+	case CMD_insert:
+	case CMD_append:
+	case CMD_change:
+	case CMD_xit:
+	    semsg(_("E1100: Missing :let: %s"), eap->cmd);
+	    return FAIL;
+	default: break;
+    }
+    return OK;
+}
+
 /*
  * ":export let Name: type"
  * ":export const Name: type"
  * ":export def Name(..."
  * ":export class Name ..."
- *
- * ":export {Name, ...}"
  */
     void
 ex_export(exarg_T *eap)