LibWeb: Start implementing <input type=text> using a shadow DOM

Text <input> fields will now generate a basic shadow DOM and attach it
to the input element.

The shadow DOM contains a <div> with some inline style, and an always-
editable text node inside it. Accessing the "value" attribute on such
an input element will get/set the value from that text node.

This is really cool, although not super stable since HTML editing is
not super stable. But it's a start! :^)

Userland/Libraries/LibWeb/HTML/HTMLInputElement.idl

@@ -11,6 +11,8 @@ interface HTMLInputElement : HTMLElement {
     [Reflect=dirname] attribute DOMString dirName;
     [Reflect=value] attribute DOMString defaultValue;
 
+    [LegacyNullToEmptyString] attribute DOMString value;
+
     attribute boolean checked;
 
     [Reflect] attribute boolean disabled;