Throw helpful error if `privateKey` is set to only the first line, e.g. `-----BEGIN RSA PRIVATE KEY-----`

Follow up to https://github.com/octokit/auth-app.js/issues/465#issuecomment-1564998327, reported by @reza-ebrahimi

The first line might be `-----BEGIN PRIVATE KEY-----` or `-----BEGIN RSA PRIVATE KEY-----`, maybe we will support other variations in future. But they all have the format `-----BEGIN * KEY-----`

I would suggest we add a check like

```js
if (/-----BEGIN * KEY-----/.test(privateKey?.trim()) {
throw new Error("privateKey only contains the first line. Try replacing line breaks with \n")
}
```