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.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2022, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/HashMap.h>
+#include <LibXML/DOM/Node.h>
+
+namespace XML {
+
+bool Node::operator==(Node const& other) const
+{
+    return content.visit(
+        [&](Text const& text) -> bool {
+            auto other_text = other.content.get_pointer<Text>();
+            if (!other_text)
+                return false;
+            return text.builder.string_view() == other_text->builder.string_view();
+        },
+        [&](Comment const& comment) -> bool {
+            auto other_comment = other.content.get_pointer<Comment>();
+            if (!other_comment)
+                return false;
+            return comment.text == other_comment->text;
+        },
+        [&](Element const& element) -> bool {
+            auto other_element = other.content.get_pointer<Element>();
+            if (!other_element)
+                return false;
+            if (element.name != other_element->name)
+                return false;
+            if (element.attributes.size() != other_element->attributes.size())
+                return false;
+
+            for (auto& entry : element.attributes) {
+                auto it = other_element->attributes.find(entry.key);
+                if (it == other_element->attributes.end())
+                    return false;
+                if (it->value != entry.value)
+                    return false;
+            }
+
+            if (element.children.size() != other_element->children.size())
+                return false;
+            for (size_t i = 0; i < element.children.size(); ++i) {
+                if (element.children[i] != other_element->children[i])
+                    return false;
+            }
+            return true;
+        });
+}
+
+}