LibWeb: Implement the color-scheme meta tag name

Author: https://github.com/Gingeh
Commit: df70455d3f
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3146
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/DOM/Document.h

@@ -242,6 +242,9 @@ public:
     Optional<Color> visited_link_color() const;
     void set_visited_link_color(Color);
 
+    Optional<Vector<String> const&> supported_color_schemes() const;
+    void obtain_supported_color_schemes();
+
     void update_style();
     void update_layout();
     void update_paint_and_hit_testing_properties_if_needed();
@@ -829,6 +832,8 @@ private:
     Optional<Color> m_active_link_color;
     Optional<Color> m_visited_link_color;
 
+    Optional<Vector<String>> m_supported_color_schemes;
+
     GC::Ptr<HTML::HTMLParser> m_parser;
     bool m_active_parser_was_aborted { false };