add notify skill

[billy-thegreat]

This pull request adds a new notification skill. Still working on exposing metadata like source (e.g., text message, email) to the user. Future PRs will improve UI by creating distinction between notifications and improving spoken strings and input sentences.

[Stypox]

Thank you! I rebased the changes on master, but now I can't push to your branch because you disallowed it, so for now I pushed them here. Could you close this Pull Request and open another one, and leave the following checkbox ticked when you open it?