image.toDataURL() is missing

[lpatiny]

I could not find the way to convert an image toDataURL in the new library

Previous implementation:

https://github.com/image-js/image-js/blob/17d7e83096318901d998d2139d974a2237af137c/src/image/core/export.js#L121-L143

https://github.com/image-js/image-js/blob/legacy/src/image/core/__tests__/toDataURL.js