LibWeb: Add basic support for <input type=checkbox>

This is implemented entirely inside LibWeb, there is no GUI::CheckBox
widget instantiated, unlike other input types. All input types should
be moved to this new style of implementation.

Libraries/LibWeb/HTML/HTMLInputElement.idl

@@ -11,4 +11,5 @@ interface HTMLInputElement : HTMLElement {
     [Reflect=dirname] attribute DOMString dirName;
     [Reflect=value] attribute DOMString defaultValue;
 
+    attribute boolean checked;
 }