LibWeb: Add HTMLButtonElement#disabled

Userland/Libraries/LibWeb/HTML/HTMLButtonElement.idl

@@ -6,6 +6,7 @@ interface HTMLButtonElement : HTMLElement {
 
     [HTMLConstructor] constructor();
 
+    [CEReactions, Reflect] attribute boolean disabled;
     [CEReactions, Reflect=formnovalidate] attribute boolean formNoValidate;
     [CEReactions, Reflect=formtarget] attribute DOMString formTarget;
     [CEReactions, Reflect] attribute DOMString name;