patch 8.2.1257: Vim9: list unpack doesn't work at the script level

Problem:    Vim9: list unpack doesn't work at the script level.
Solution:   Detect unpack assignment better. (closes #6494)
diff --git a/src/version.c b/src/version.c
index ee1e3f4..cc7fbfb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1257,
+/**/
     1256,
 /**/
     1255,