patch 8.2.4509: Vim9: can declare a variable with ":va"

Problem:    Vim9: can declare a variable with ":va".
Solution:   Disallow using ":va", require using ":var".
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 8987123..52c60fb 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1883,7 +1883,14 @@
     lhs_T	lhs;
     long	start_lnum = SOURCING_LNUM;
 
-    // Skip over the "var" or "[var, var]" to get to any "=".
+    p = eap->cmd;
+    if (eap->cmdidx == CMD_var && arg > p && !checkforcmd_noparen(&p, "var", 3))
+    {
+	emsg(_(e_must_use_var_instead_of_va));
+	return NULL;
+    }
+
+    // Skip over the "varname" or "[varname, varname]" to get to any "=".
     p = skip_var_list(arg, TRUE, &var_count, &semicolon, TRUE);
     if (p == NULL)
 	return *arg == '[' ? arg : NULL;