Enable adding modifiers without changing InterpolationRenderer

The `InterpolationRenderer` was violating the open-closed principle,
because every time we would want to add a new modifier, we would need to
change its implementation.

This commit changes that behaviour by creating a `Modifier` interface.
The classes implementing that interface are using a chain of
responsibility to pass the data to the next one. Using a chain of
responsibility makes a lot of sense, since it's only possible to have
one modifier at a time.

tests/library/Message/TestingModifier.php

@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * Copyright (c) Alexandre Gomes Gaigalas <alganet@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+declare(strict_types=1);
+
+namespace Respect\Validation\Test\Message;
+
+use Respect\Validation\Message\Modifier;
+
+use function print_r;
+use function sprintf;
+
+final readonly class TestingModifier implements Modifier
+{
+    public function modify(mixed $value, string|null $pipe): string
+    {
+        return $pipe ? sprintf('%s(%s)', $pipe, print_r($value, true)) : print_r($value, true);
+    }
+}