LibWeb: Start implementing character token parsing

Now that we've gotten rid of the misguided character buffering in the
tokenizer, it actually spits out character tokens that we have to deal
with in the parser.

This patch implements enough to bring us back to speed with simple.html

Libraries/LibWeb/DOM/CharacterData.h

@@ -36,6 +36,7 @@ public:
     virtual ~CharacterData() override;
 
     const String& data() const { return m_data; }
+    void set_data(const String& data) { m_data = data; }
 
     virtual String text_content() const override { return m_data; }