Prepare for upcoming deprecation of configuring diagnostic-signs using sign_define()

[glmlm]

Since Neovim 0.10, it seems that configuring diagnostic-signs using sign_define() has been deprecated.
There is another way to set them as follows:

if vim.g.have_nerd_font then
  vim.diagnostic.config {
    signs = {
      text = {
        [vim.diagnostic.severity.ERROR] = '',
        [vim.diagnostic.severity.WARN] = '',
        [vim.diagnostic.severity.INFO] = '',
        [vim.diagnostic.severity.HINT] = '',
      },
    },
  }
end

In the committed one, I simply focused on fewer lines and readability(after all, this section is commented out).

[feoh]

Thanks for this! Merging.