LibWeb: Move contentEditable from Element to HTMLElement

HTMLElement is the only interface that includes ElementContentEditable
in the HTML specification. This makes sense, as Element is also a base
class for elements in other specifications such as SVG,
which definitely shouldn't be editable.

Also adds a test for the attribute based on what Andreas did in the
video that added it.

Libraries/LibWeb/Tests/Elements/HTML/HTMLElement.js

@@ -0,0 +1,9 @@
+loadPage("file:///res/html/misc/welcome.html")
+
+afterInitialPageLoad(() => {
+   test("contentEditable attribute", () => {
+        expect(document.body.contentEditable).toBe("inherit");
+        expect(document.firstChild.nextSibling.nodeName).toBe("html");
+        expect(document.firstChild.nextSibling.contentEditable).toBe("true");
+   });
+});