fix: define renderToStream to return promise

[dschu-lab]

The old return type was string which is wrong, because it returns a promise which resolves in a string instead.

This pull request fixes the warning 'await' has no effect on the type of this expression when using it like this:

    const document = await ReactPDF.renderToString(...)

[jeetiss]

@dschu-lab thanks for your contribution! 🙌🏻