Undo override of Tailwind classname separator for Pug

[arildm]

tailwind.config.js has:

    // pug doesn't support special characters in class names, so no colon.
    separator: "_",

See https://tailwindcss.com/docs/configuration#separator

Pug was removed in February 2023 (b13084b), so there's no need for that anymore.

Switching back shouldn't be a huge commit, and should be easier in the long run.