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.

まず、CodeWorld の構文ハイライトのエントリーポイントと、Prelude の識別子と他のライブラリやユーザーコードから提供される名前を区別しているロジックを見つけます。定義済み識別子に対する特別なハイライトを削除し、Double、True、Event、Picture、id などの型や関数が他の識別子と同じように扱われることを確認します。

索引モデルが issue の本文から書いたものです。