LibWeb: Factor out the "stack of open elements" into its own class

This will allow us to write more expressive parsing code. :^)

Libraries/LibWeb/CMakeLists.txt

@@ -88,6 +88,7 @@ set(SOURCES
     Parser/HTMLParser.cpp
     Parser/HTMLToken.cpp
     Parser/HTMLTokenizer.cpp
+    Parser/StackOfOpenElements.cpp
     ResourceLoader.cpp
     StylePropertiesModel.cpp
     URLEncoder.cpp