vfmt: change all '$expr' to '${expr}' (#16428)

vlib/json/json_decode_with_optional_arg_test.v

@@ -15,5 +15,5 @@ fn test_json_decode_with_optional_arg() {
 fn print_info() !string {
 	dbconf := json.decode(DbConfig, os.read_file('dbconf.json')!)!
 	println(dbconf)
-	return '$dbconf'
+	return '${dbconf}'
 }