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/cmdexpand.c b/src/cmdexpand.c
index e7cf819..f3e19bc 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -1513,8 +1513,10 @@
 	    break;
 #endif
 #ifdef FEAT_EVAL
+	case CMD_final:
 	case CMD_const:
 	case CMD_let:
+	case CMD_var:
 	case CMD_if:
 	case CMD_elseif:
 	case CMD_while: