Add the shared preference keys and appearance values

tests/Id/SharedPreferenceTest.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\Enums\Appearance;
+use Anikeen\Id\Enums\SharedPreference;
+
+class SharedPreferenceTest extends TestCase
+{
+    public function test_the_shared_keys_and_values_match_anikeen_id(): void
+    {
+        $this->assertSame('global', SharedPreference::NAMESPACE);
+        $this->assertSame('appearance', SharedPreference::APPEARANCE);
+        $this->assertSame(['system', 'light', 'dark'], Appearance::all());
+    }
+}