Add support for Polish language stemming

[asdfMaciej]

Hello! Tantivy is used by multiple projects - I've found it as a dependency of ParadeDB. I would love it if it supported stemming in Polish language, as it would enable Polish language support downstream.

I'm not sure though if it's currently possible? I saw that tantivy uses rust_stemmers as a dependency for multi-language stemming. However, its repository appears to be unmaintained with 13 open PRs and the last commit being from 2021. Also, it appears that a language needs to be added to Snowball prior to adding to rust_stemmers, which has an open PR (since 2021) for the Polish language: snowballstem/snowball#159

Sadly, I don't know NLP well enough to contribute - but I hope this write-up comes in handy for someone :)

Thanks for maintaining this project!

[asdfMaciej]

I've found this crate, which appears to be maintained and tackles the same exact issue: https://github.com/testuj-to/tantivy-stemmers