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/Document.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2022, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/NonnullOwnPtr.h>
+#include <LibXML/DOM/DocumentTypeDeclaration.h>
+#include <LibXML/DOM/Node.h>
+#include <LibXML/Forward.h>
+
+namespace XML {
+
+enum class Version {
+    Version10,
+    Version11,
+};
+
+struct Doctype {
+    String type;
+    Vector<MarkupDeclaration> markup_declarations;
+    Optional<ExternalID> external_id;
+};
+
+class Document {
+public:
+    explicit Document(NonnullOwnPtr<Node> root, Optional<Doctype> doctype, HashMap<Name, String> processing_instructions, Version version)
+        : m_root(move(root))
+        , m_processing_instructions(move(processing_instructions))
+        , m_version(version)
+        , m_explicit_doctype(move(doctype))
+    {
+    }
+
+    Node& root() { return *m_root; }
+    Node const& root() const { return *m_root; }
+
+    HashMap<Name, String> const& processing_instructions() const { return m_processing_instructions; }
+
+    Version version() const { return m_version; }
+
+    Optional<Doctype> const& doctype() const { return m_explicit_doctype; }
+
+private:
+    NonnullOwnPtr<Node> m_root;
+    HashMap<Name, String> m_processing_instructions;
+    Version m_version;
+    Optional<Doctype> m_explicit_doctype;
+};
+}