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.

Meta/CMake/all_the_debug_macros.cmake

@@ -209,6 +209,7 @@ set(WEB_WORKER_DEBUG ON)
 set(WINDOWMANAGER_DEBUG ON)
 set(WSMESSAGELOOP_DEBUG ON)
 set(WSSCREEN_DEBUG ON)
+set(XML_PARSER_DEBUG ON)
 
 # False positive: DEBUG is a flag but it works differently.
 # set(DEBUG ON)

Meta/Lagom/CMakeLists.txt

@@ -478,6 +478,10 @@ if (BUILD_LAGOM)
         )
     endif()
 
+    file(GLOB LIBXML_SOURCES CONFIGURE_DEPENDS "../../Userland/Libraries/LibXML/*/*.cpp")
+    lagom_lib(XML xml
+        SOURCES ${LIBXML_SOURCES})
+
     if (NOT ENABLE_OSS_FUZZ AND NOT ENABLE_FUZZER_SANITIZER AND NOT ENABLE_COMPILER_EXPLORER_BUILD)
         # Lagom Examples
         add_executable(TestApp TestApp.cpp)
@@ -523,6 +527,10 @@ if (BUILD_LAGOM)
         set_target_properties(wasm_lagom PROPERTIES OUTPUT_NAME wasm)
         target_link_libraries(wasm_lagom LagomCore LagomWasm LagomLine LagomMain)
 
+        add_executable(xml_lagom ../../Userland/Utilities/xml.cpp)
+        set_target_properties(xml_lagom PROPERTIES OUTPUT_NAME xml)
+        target_link_libraries(xml_lagom LagomCore LagomXML LagomMain)
+
         enable_testing()
         # LibTest
         file(GLOB LIBTEST_SOURCES CONFIGURE_DEPENDS "../../Userland/Libraries/LibTest/*.cpp")