To ensure compatibility with the latest versions of Neovim LSP the
highlighting groups for diagnostics have been adapted to the changes
of neovim/neovim#12655 [1].
See :help lsp-highlight-diagnostics [2] for more details.

Note that LSP will be available as of Neovim 0.5 which is (at the time
of this commit) still in development and only available as nighly build.
Also see great articles from Nord Vim contributors like "Neovim (0.5) Is
Overpowering" [3] for more information about Neovim 0.5 features,
including LSP.

[1]: neovim/neovim#12655
[2]: https://neovim.io/doc/user/lsp.html
[3]: https://crispgm.com/page/neovim-is-overpowering.html

Co-authored-by: Sven Greb <development@svengreb.de>

Closes nordthemeGH-229
Closes nordthemeGH-248