patch 8.2.1744: Vim9: using ":const!" is weird

Problem:    Vim9: using ":const!" is weird.
Solution:   Use "var" - "final" - "const" like Dart.  "let" still works for
            now.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 02d2c30..955c3aa 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -729,7 +729,7 @@
     funccal_entry_T entry;
 
     save_funccal(&entry);
-    set_var_const(name, NULL, tv, FALSE, LET_NO_COMMAND);
+    set_var_const(name, NULL, tv, FALSE, ASSIGN_NO_DECL);
     restore_funccal();
 }