make link urls underline not undercurl

[JordanShurmer]

undercurl implies an error or warning, so a plain underline seems more appropriate

[pratyushmittal]

The PR looks good to me. Can we get this merged please?

Temporary solution till then (using Lazy):

  {
    'https://github.com/rebelot/kanagawa.nvim',
    opts = {
      undercurl = true,
      overrides = function(colors)
        return {
          ['@string.special.url'] = { underline = true, undercurl = false },
        }
      end,
    },
  }