Update to LSP Version 3.15

[cdietrich]

It would be nice if this module would get support for LSP version 3.15
https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/

[UziTech]

Development of atom-languageclient has officially moved to https://github.com/atom-ide-community/atom-languageclient 🎉

If this is still an issue please consider opening an issue on that repo.