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/unit/Message/Modifier/StringifyModifierTest.php

@@ -0,0 +1,63 @@
+<?php
+
+/*
+ * Copyright (c) Alexandre Gomes Gaigalas <alganet@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+declare(strict_types=1);
+
+namespace Respect\Validation\Message\Modifier;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use Respect\Validation\Exceptions\ComponentException;
+use Respect\Validation\Test\Message\NullStringifier;
+use Respect\Validation\Test\Message\TestingStringifier;
+use Respect\Validation\Test\TestCase;
+
+use function print_r;
+use function sprintf;
+
+#[CoversClass(StringifyModifier::class)]
+final class StringifyModifierTest extends TestCase
+{
+    #[Test]
+    public function itShouldUseStringifierWhenAvailable(): void
+    {
+        $value = ['some', 'array'];
+
+        $stringifier = new TestingStringifier();
+        $modifier = new StringifyModifier($stringifier);
+
+        $expected = $stringifier->stringify($value, 0);
+
+        self::assertSame($expected, $modifier->modify($value, null));
+    }
+
+    #[Test]
+    public function itShouldUseFallbackWhenStringifierIsNull(): void
+    {
+        $value = ['some', 'array'];
+        $expected = print_r($value, true);
+
+        $modifier = new StringifyModifier(new NullStringifier());
+
+        self::assertSame($expected, $modifier->modify($value, null));
+    }
+
+    #[Test]
+    public function itShouldFailWhenPipeParameterIsGiven(): void
+    {
+        $pipe = 'someModifier';
+
+        $modifier = new StringifyModifier(new TestingStringifier());
+
+        $this->expectExceptionObject(new ComponentException(sprintf(
+            'StringifyModifier only accepts null as  pipe but "%s" was given.',
+            $pipe,
+        )));
+
+        $modifier->modify(['some', 'array'], $pipe);
+    }
+}