feat: format() takes an optional callback

[stevearc]

closes #19

Enables async format-on-save workflow that re-saves the buffer after formatting completes.

vim.api.nvim_create_autocmd("BufWritePost", {
  pattern = "*",
  callback = function(args)
    require("conform").format({ async = true, lsp_fallback = true, buf = args.buf }, function(err)
      if not err then
        vim.api.nvim_buf_call(args.buf, function()
          vim.cmd.update()
        end)
      end
    end)
  end,
})