LSP: enable using different highlighting rules for LSP signs (#12176)

This commit creates 4 new highlight groups:
- LspDiagnosticsErrorSign
- LspDiagnosticsWarningSign
- LspDiagnosticsInformationSign
- LspDiagnosticsHintSign

These highlight groups are linked to their corresponding LspDiagnostics
highlight groups by default.

This lets users choose a different color for their sign columns and
virtualtext diagnostics.

runtime/lua/vim/lsp/util.lua

@@ -672,6 +672,7 @@ do
       table.insert(cmd_parts, k.."="..v)
     end
     api.nvim_command(table.concat(cmd_parts, ' '))
+    api.nvim_command('highlight link ' .. highlight_name .. 'Sign ' .. highlight_name)
     severity_highlights[severity] = highlight_name
   end