LibWeb: Add implementation of Node.compareDocumentPosition()

While looking into getting Duck Duck Go loading further in the
Browser, I noticed that it was complaining about the missing
method Node.compareDocumentPosition.

This change implements as much of the DOM spec as possible
with the current implementation of the DOM to date. The
implementation is validated by new tests in the Node.js.

Userland/Libraries/LibWeb/Tests/DOM/Node.js

@@ -36,4 +36,27 @@ afterInitialPageLoad(() => {
         document.body.removeChild(element);
         expect(element.isConnected).toBeFalse();
     });
+
+    test("Node.compareDocumentPosition()", () => {
+        const head = document.head;
+        const body = document.body;
+
+        expect(head.compareDocumentPosition(head)).toBe(0);
+
+        // FIXME: Can be uncommented once the IDL parser correctly implements nullable paramaters.
+        // expect(head.compareDocumentPosition(null) & Node.DOCUMENT_POSITION_DISCONNECTED | Node.DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC).
+        //    toBe(Node.DOCUMENT_POSITION_DISCONNECTED | Node.DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC);
+
+        expect(head.compareDocumentPosition(body)).toBe(Node.DOCUMENT_POSITION_FOLLOWING);
+        expect(body.compareDocumentPosition(head)).toBe(Node.DOCUMENT_POSITION_PRECEDING);
+
+        const source = document.getElementById("source");
+        expect(source.compareDocumentPosition(body)).toBe(
+            Node.DOCUMENT_POSITION_CONTAINS | Node.DOCUMENT_POSITION_PRECEDING
+        );
+        expect(body.compareDocumentPosition(source)).toBe(
+            Node.DOCUMENT_POSITION_CONTAINED_BY | Node.DOCUMENT_POSITION_FOLLOWING
+        );
+        expect(source.compareDocumentPosition(head)).toBe(Node.DOCUMENT_POSITION_PRECEDING);
+    });
 });