LibWeb: Add motion preference

This adds a motion preference to the browser UI similar to the existing
ones for color scheme and contrast.
Both AppKit UI and Qt UI has this new preference.
The auto value is currently the same as NoPreference, follow-ups can
address wiring that up to the actual preference for the OS.

Userland/Libraries/LibWeb/CSS/PreferredMotion.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2024, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+
+namespace Web::CSS {
+
+enum class PreferredMotion {
+    Auto,
+    NoPreference,
+    Reduce,
+};
+
+PreferredMotion preferred_motion_from_string(StringView);
+StringView preferred_motion_to_string(PreferredMotion);
+
+}