Add file extensions to language from "Settings"

[scimax]

I think it would be a nice option to add custom file extensions to the association of languages from the "Settings" menu. I'm not used to the code base but @ian-r-rose suggested jupyterlab_settings and jupyterlab_workspaces for this purpose in #3858.

[saulshanabrook]

I think #4223 is about the same thing, so closing this in favor of that.