LibWeb: Implement :required/:optional pseudo-classes

Author: https://github.com/Gingeh
Commit: 7acc0f4a42
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4838
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -219,6 +219,7 @@ public:
     bool selection_direction_applies() const;
     bool pattern_applies() const;
     bool multiple_applies() const;
+    bool required_applies() const;
     bool has_selectable_text() const;
 
     bool supports_a_picker() const;