Sunsetting Atom

[ggrossetie]

GitHub is sunsetting Atom on December 15, 2022

Thanks to all contributors ❤️, it's time to move on 🥲

[ggrossetie]

Once this merge request is approved, I will use the same message on:

• https://github.com/asciidoctor/atom-language-asciidoc
• https://github.com/asciidoctor/atom-asciidoc-assistant
• https://github.com/asciidoctor/atom-asciidoc-image-helper
• https://github.com/asciidoctor/atom-autocomplete-asciidoc