Add option to show the most severe diagnostics per line

[ompugao]

I'm unsure why the original author of #1420 closed the issue, but I believe this option is still necessary. Without it, the new lines inserted by the virtual texts of diagnostics make the view messy and difficult to track at a glance.

Additionally, I'm unclear on how the option let g:lsp_diagnostics_virtual_text_severities = [1,2,3...], as mentioned in the comment , would work with this feature. I'd like to continue the discussion on this.