feat: add YesNoMaybeGrammar

src/BackusNaurFormGrammar/YesNoMaybeGrammar.php

+<?php
+
+declare(strict_types=1);
+
+namespace Distantmagic\Resonance\BackusNaurFormGrammar;
+
+use Distantmagic\Resonance\Attribute\Singleton;
+use Distantmagic\Resonance\BackusNaurFormGrammar;
+
+#[Singleton]
+readonly class YesNoMaybeGrammar extends BackusNaurFormGrammar
+{
+    /**
+     * @var non-empty-string
+     */
+    private string $grammar;
+
+    public function __construct()
+    {
+        $this->grammar = 'root ::= "yes" | "no" | "maybe"';
+    }
+
+    public function getGrammarContent(): string
+    {
+        return $this->grammar;
+    }
+}