No special syntax highlighting for common identifiers

This is a bit of a bikeshed issue, but I’d like to raise it anyways. Feel free to close without much discussion if you disagree.

Currently, CodeWorld syntax highlighting highlights names that happen to be defined in the prelude library (`Double`, `True`) different that names from other libraries (`Event`, `Picture`) or custom-defined things. Similar for function (e.g. `id`).

I generally disapprove of this approach, as it hides the fact that even the Prelude is just a library, and that `Double` and `Event` are both types of their own right. The difference between the two is not worth this distinction, and the type of, say `interactionOf`, looks bad with these types printed differently.

Also, the `Prelude` changes over time, making this highlighting inconsistent.

I’d like to see no special highlighting of predefined identifiers.