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/vim9script.c b/src/vim9script.c
index 74d0579..399c4a7 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -97,6 +97,8 @@
     switch (eap->cmdidx)
     {
 	case CMD_let:
+	case CMD_var:
+	case CMD_final:
 	case CMD_const:
 	case CMD_def:
 	// case CMD_class:
@@ -508,9 +510,12 @@
     int		    called_emsg_before = called_emsg;
     typval_T	    init_tv;
 
-    if (eap->cmdidx == CMD_const)
+    if (eap->cmdidx == CMD_final || eap->cmdidx == CMD_const)
     {
-	emsg(_(e_const_requires_a_value));
+	if (eap->cmdidx == CMD_final)
+	    emsg(_(e_final_requires_a_value));
+	else
+	    emsg(_(e_const_requires_a_value));
 	return arg + STRLEN(arg);
     }