Add support for smooth-scrolling package

[trevorpogue]

Resolves #13

• The smooth-scrolling package / smooth-scrolling-mode can now be used with topspce-mode without unexpected behaviour.

---

Checklist

• I have read the topspace contributing guidelines
• My changes follow the Emacs Lisp conventions and the Emacs Lisp Style Guide
• I've used the latest version of package-lint to check for packaging issues, and addressed its feedback
• The new code is not generating bytecode warnings
• I've updated the readme (if adding/changing user-visible functionality)
• I have confirmed some of these without doing them