feat: Refactor mention rendering and generator classes

- Add support for Mastodon username mentions
- Add test for parsing Mastodon usernames in notes
- Modify namespace and class imports for `MentionGenerator` and `MentionRenderer` in `Note.php`
- Rename `ContactMentionGenerator.php` to `MentionGenerator.php`

app/CommonMark/Generators/MentionGenerator.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\CommonMark\Generators;
+
+use League\CommonMark\Extension\Mention\Generator\MentionGeneratorInterface;
+use League\CommonMark\Extension\Mention\Mention;
+use League\CommonMark\Node\Inline\AbstractInline;
+
+class MentionGenerator implements MentionGeneratorInterface
+{
+    public function generateMention(Mention $mention): ?AbstractInline
+    {
+        return $mention;
+    }
+}