This moves some stuff around to make LibGUI depend on LibSyntax instead
of the other way around, as not every application that wishes to do
syntax highlighting is necessarily a LibGUI (or even a GUI) application.
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.)
This works at the Token level, which is quick and easy but has
drawbacks: We don't know when something is a property name or a value,
or if something is part of a selector. But, this works for now.
