patch 8.2.2157: Vim9: can delete a Vim9 script variable from a function

Problem:    Vim9: can delete a Vim9 script variable from a function.
Solution:   Check the variable is defined in Vim9 script. (closes #7483)
diff --git a/src/version.c b/src/version.c
index be2b33c..70ff9a4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2157,
+/**/
     2156,
 /**/
     2155,