LibWeb+LibJS: Move some code around to make CSS/Parser parse faster

This makes it possible to include fewer full definitions of things,
which makes the file about 30% faster to compile.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -54,6 +54,7 @@ set(SOURCES
     CSS/Parser/DeclarationOrAtRule.cpp
     CSS/Parser/Function.cpp
     CSS/Parser/Parser.cpp
+    CSS/Parser/ParsingContext.cpp
     CSS/Parser/Rule.cpp
     CSS/Parser/Token.cpp
     CSS/Parser/Tokenizer.cpp