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.

library/Message/Modifier/RawModifier.php

@@ -0,0 +1,36 @@
+<?php
+
+/*
+ * Copyright (c) Alexandre Gomes Gaigalas <alganet@gmail.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+declare(strict_types=1);
+
+namespace Respect\Validation\Message\Modifier;
+
+use Respect\Validation\Message\Modifier;
+
+use function is_bool;
+use function is_scalar;
+
+final readonly class RawModifier implements Modifier
+{
+    public function __construct(
+        private Modifier $nextModifier,
+    ) {
+    }
+
+    public function modify(mixed $value, string|null $pipe): string
+    {
+        if ($pipe !== 'raw') {
+            return $this->nextModifier->modify($value, $pipe);
+        }
+
+        if (!is_scalar($value)) {
+            return $this->nextModifier->modify($value, null);
+        }
+
+        return is_bool($value) ? (string) (int) $value : (string) $value;
+    }
+}