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)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 987e92d..fe65410 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -5686,6 +5686,8 @@
     static void
 ex_exit(exarg_T *eap)
 {
+    if (not_in_vim9(eap) == FAIL)
+	return;
 #ifdef FEAT_CMDWIN
     if (cmdwin_type != 0)
     {