LibWeb: Add Node.textContent

This requires moving remove_all_children() from ParentNode to
Node, which makes ParentNode.cpp empty, so remove it.

It also co-opts the existing Node::text_content() method and
tweaks it slightly to fit the semantics of Node.textContent.

Libraries/LibWeb/Tests/DOM/Node.js

@@ -0,0 +1,20 @@
+loadPage("file:///res/html/misc/small.html");
+
+afterInitialPageLoad(() => {
+    test("Node.textContent", () => {
+        var p = document.getElementsByTagName("p")[0];
+        expect(p.textContent).toBe("This is a very small test page :^)");
+        expect(p.firstChild.textContent).toBe("This is a ");
+        expect(p.firstChild.firstChild).toBe(null);
+
+        p.firstChild.textContent = "foo";
+        expect(p.firstChild.textContent).toBe("foo");
+        expect(p.firstChild.firstChild).toBe(null);
+        expect(p.textContent).toBe("foovery small test page :^)");
+
+        p.textContent = "bar";
+        expect(p.textContent).toBe("bar");
+        expect(p.firstChild.textContent).toBe("bar");
+        expect(p.firstChild.firstChild).toBe(null);
+    });
+});