LibWeb: Implement "plaintext-only" state for contenteditable

Author: https://github.com/gmta
Commit: 217567981a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2697
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/tcl3

Libraries/LibWeb/HTML/HTMLElement.h

@@ -105,9 +105,11 @@ private:
     // https://html.spec.whatwg.org/multipage/custom-elements.html#attached-internals
     GC::Ptr<ElementInternals> m_attached_internals;
 
+    // https://html.spec.whatwg.org/#attr-contenteditable
     enum class ContentEditableState {
         True,
         False,
+        PlaintextOnly,
         Inherit,
     };
     ContentEditableState m_content_editable_state { ContentEditableState::Inherit };