LibWeb: Extract the TextEncoderCommon mixin to its own IDL file

Co-Authored-By: Tim Flynn <trflynn89@pm.me>

Libraries/LibWeb/Encoding/TextEncoderCommon.cpp

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Encoding/TextEncoderCommon.h>
+
+namespace Web::Encoding {
+
+TextEncoderCommonMixin::TextEncoderCommonMixin() = default;
+TextEncoderCommonMixin::~TextEncoderCommonMixin() = default;
+
+// https://encoding.spec.whatwg.org/#dom-textencoder-encoding
+FlyString const& TextEncoderCommonMixin::encoding() const
+{
+    // The encoding getter steps are to return "utf-8".
+    static FlyString const encoding = "utf-8"_fly_string;
+    return encoding;
+}
+
+}