feat: allow disabling builtin UI in favor of vim.notify

[moyiz]

Adds a new configuration option to allow disabling UI.

To disable:

{
  ui = {
    enabled = false,
  }
}

It will fallback to vim.notify and will show two notifications:

1. Opening ... - When calling open.
2. An aggregated notification of all output from this point until open exits.

Closes #26