LibWeb: Propagate errors from CSS Tokenizer construction

Instead of constructing a Tokenizer and then calling parse() on it, we
now call `Tokenizer::tokenize(...)` directly. (Renamed from `parse()`
because this is a Tokenizer, not a Parser.)

Userland/Libraries/LibWeb/CSS/Parser/Parser.h

@@ -360,7 +360,6 @@ private:
 
     ParsingContext m_context;
 
-    Tokenizer m_tokenizer;
     Vector<Token> m_tokens;
     TokenStream<Token> m_token_stream;
 };