fix(lsp): use distinct styles for (non-)underline diags

[antoineco]

Supersedes #136 Fixes https://github.com/sainnhe/gruvbox-material/issues/120 Fixes https://github.com/sainnhe/sonokai/issues/87 Fixes https://github.com/sainnhe/everforest/issues/121 (proper fix, not upstream workaround) Fixes https://github.com/sainnhe/gruvbox-material/issues/145 (proper fix, not upstream workaround)

Description

Ensures that LspDiagnostic and LspDiagnosticUnderline use distinct styles: highlighted for the former, underline (undercurl) for the latter.

Rationale:

• If underline is enabled in Neovim, the editor will always use the LspDiagnosticUnderline variant.
• By setting a foreground color for the LspDiagnostic variant, we ensure that the many popular plugins which leverage these highlight groups default to the correct foreground color.

I have used these settings for a bit and tried them with plugins that used to have issues with Everforest. So far I haven't noticed any problem.

Screenshots

Nothing new to see here, but I'm including examples of highlights without/with g:everforest_diagnostic_text_highlight to illustrate the PR:

[antoineco]

@zennolux would you by any chance be able to test this branch with LSP Saga?

[zennolux]

@antoineco

Great! It works!

Thank you for your work.