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/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(SOURCES
+    Parser/Parser.cpp
+    DOM/Node.cpp
+)
+
+serenity_lib(LibXML xml)
+target_link_libraries(LibXML LibC)