Add support for more languages and providers

- We can cheaply add basic support for numerous languages by implementing generic fetchers for sites like Tatoeba.
- We should utilize existing libraries and vendorize them - the existing Make-based build system will probably become unwieldy after we add more than a couple of dependencies.

Contributions to add more languages and providers are welcome! See https://github.com/abdnh/anki-incontext/issues/1#issuecomment-1091911041 as a starting point.