feat: export heic to jpg

[lubbo]

It may be useful to always export .jpg... I'm using Plex photo (shame on me!) and it doesn't support .heic

[abentele]

Thanks @lubbo for implementing this feature and providing it via this pull request.

While I find this feature really useful, the implementation would produce duplicate files in the different folders ("Originals"/"Derived"/"Current"). The philosophy of the PhotosExporter is to use hard links for files which are the same as much as possible, to save disk space. If you convert a heic to jpg in the "Originals" folder, the same heic to jpg conversion is applied for the "Derived" folder and to the "Current" folder, instead of creating hard links to the same jpg file.

Would you like to modify your code in this direction? I would appreciate to integrate it then into the master branch.