builtin: change IError msg and code to methods + fix vlib, add a deprecation notice for the old usages (#13041)

vlib/x/json2/decoder.v

@@ -22,13 +22,11 @@ mut:
 }
 
 struct InvalidTokenError {
-	msg  string
-	code int
+	MessageError
 }
 
 struct UnknownTokenError {
-	msg  string
-	code int
+	MessageError
 }
 
 fn (mut p Parser) next() {

vlib/x/json2/decoder_test.v

@@ -43,7 +43,7 @@ fn test_raw_decode_null() ? {
 
 fn test_raw_decode_invalid() ? {
 	raw_decode('1z') or {
-		assert err.msg == '[x.json2] invalid token `z` (0:17)'
+		assert err.msg() == '[x.json2] invalid token `z` (0:17)'
 		return
 	}
 	assert false