LibXML: Add a fairly basic XML parser

Currently this can parse XML and resolve external resources/references,
and read a DTD (but not apply or verify its rules).
That's good enough for _most_ XHTML documents as the HTML 5 spec
enforces its own rules about document well-formedness, and does not make
use of XML DTDs (aside from a list of predefined entities).

An accompanying `xml` utility is provided that can read and dump XML
documents, and can also run the XML conformance test suite.

Userland/Libraries/LibXML/DOM/Node.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2022, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtrVector.h>
+#include <AK/String.h>
+#include <AK/Variant.h>
+#include <AK/Vector.h>
+#include <LibXML/FundamentalTypes.h>
+
+namespace XML {
+
+struct Attribute {
+    Name name;
+    String value;
+};
+
+struct Node {
+    struct Text {
+        StringBuilder builder;
+    };
+    struct Comment {
+        String text;
+    };
+    struct Element {
+        Name name;
+        HashMap<Name, String> attributes;
+        NonnullOwnPtrVector<Node> children;
+    };
+
+    bool operator==(Node const&) const;
+
+    Variant<Text, Comment, Element> content;
+    Node* parent { nullptr };
+};
+}