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/QuoteModifierTest.php

@@ -0,0 +1,65 @@
+<?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\Message\Placeholder\Quoted;
+use Respect\Validation\Test\Message\TestingModifier;
+use Respect\Validation\Test\TestCase;
+
+#[CoversClass(QuoteModifier::class)]
+final class QuoteModifierTest extends TestCase
+{
+    #[Test]
+    public function itShouldNotModifyWhenModifierIsNotQuote(): void
+    {
+        $nextModifier = new TestingModifier();
+        $modifier = new QuoteModifier($nextModifier);
+
+        $value = 'some string';
+        $pipe = 'notQuote';
+
+        $result = $modifier->modify($value, $pipe);
+
+        self::assertSame($nextModifier->modify($value, $pipe), $result);
+    }
+
+    #[Test]
+    public function itShouldNotModifyWhenValueIsNotString(): void
+    {
+        $nextModifier = new TestingModifier();
+        $modifier = new QuoteModifier($nextModifier);
+
+        $value = ['not', 'a', 'string'];
+        $pipe = 'quote';
+
+        $result = $modifier->modify($value, $pipe);
+
+        self::assertSame($nextModifier->modify($value, $pipe), $result);
+    }
+
+    #[Test]
+    public function itShouldModifyWhenModifierIsQuoteAndValueIsString(): void
+    {
+        $nextModifier = new TestingModifier();
+        $modifier = new QuoteModifier($nextModifier);
+
+        $value = 'some string';
+        $pipe = 'quote';
+
+        $result = $modifier->modify($value, $pipe);
+
+        $expectedValue = new Quoted($value);
+        $expected = $nextModifier->modify($expectedValue, null);
+
+        self::assertSame($expected, $result);
+    }
+}