Group related codes into categories

This patch groups all colors together in `ansi.colors`, all modifiers in `ansi.modifiers`, and all background colors in `ansi.bgColors`. The individual values still remain in their original locations, but new shortcuts emerge: `ansi.green == ansi.colors.green`.

It finally makes it possible to iterate over all available colors or modifiers.

This check looks moot, the only top-level items are objects.