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/CSS/Default.css

@@ -194,3 +194,10 @@ ul,
 ol {
     padding-left: 20px;
 }
+
+/* FIXME: This is a temporary hack until we can render a native-looking frame for these. */
+input[type=text] {
+    border: 1px solid black;
+    min-width: 80px;
+    min-height: 16px;
+}