Revert "nvim: init: Do not use underline for diagnostic"

This reverts commit 572ff25150.

nvim/.config/nvim/init.lua

@@ -87,7 +87,7 @@ vim.g.ts_highlight_lua = true
 vim.cmd.set{ args = { 'rtp-=/usr/share/vim/vimfiles' } }
 -- Disable virtual text for all diagnostics
 vim.diagnostic.config({
-  underline        = false,
+  underline        = true,
   signs            = true,
   virtual_text     = false,
   update_in_insert = false,