LibWeb: Import new CSS parser

It's not finished or hooked up to anything yet, but let's get it into
the tree so we can continue working on it.

Original work by @stelar7.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -19,6 +19,7 @@ set(SOURCES
     CSS/DefaultStyleSheetSource.cpp
     CSS/Length.cpp
     CSS/Parser/DeprecatedCSSParser.cpp
+    CSS/Parser/Parser.cpp
     CSS/Parser/StyleRules.cpp
     CSS/Parser/Token.cpp
     CSS/Parser/Tokenizer.cpp