LibWeb: Implement Element.outerHTML

This piggybacks on the same fragment serialization code that innerHTML
uses, but instead of constructing an imaginary parent element like the
spec asks us to, we just add a separate serialization mode that includes
the context element in the serialized markup.

This makes the image carousel on https://utah.edu/ show up :^)

Tests/LibWeb/Text/input/HTML/outerHTML.html

@@ -0,0 +1,8 @@
+<script src="../include.js"></script>
+<div id="foo"><b>hello students</b></div>
+<script>
+    test(() => {
+        println("outerHTML: " + foo.outerHTML)
+        println("innerHTML: " + foo.innerHTML)
+    });
+</script>