LibUnicode: Move Unicode-aware case transformations to a helper file

These will be needed by AK::String as well, so move them to a helper
file where they can be re-used.

Userland/Libraries/LibUnicode/UnicodeUtils.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/Forward.h>
+#include <AK/Optional.h>
+#include <AK/Utf8View.h>
+#include <LibUnicode/Forward.h>
+
+namespace Unicode::Detail {
+
+ErrorOr<void> build_lowercase_string(Utf8View code_points, StringBuilder& builder, Optional<StringView> const& locale);
+ErrorOr<void> build_uppercase_string(Utf8View code_points, StringBuilder& builder, Optional<StringView> const& locale);
+
+}