Add APIs using Result with thiserror or anyhow crates instead of panic

[tats-u]

https://github.com/search?q=repo%3APSeitz%2Fwana_kana_rust%20panic&type=code

This library can panic and discourages other software from adopting this library.

#13

e.g. https://github.com/orgs/meilisearch/discussions/532#discussioncomment-9999921

https://docs.rs/thiserror/latest/thiserror/ and https://docs.rs/anyhow/latest/anyhow/ are widely used to handle errors without panic.

New APIs like to_kana_safe() and to_romaji_safe() will be appreciated.

[PSeitz]

The library should never panic. Error's don't make sense imo, since it works on a best effort basis.

Here's how I addressed the issue:

1. Fixed the original issue
2. Added some proptests with random and semi random input to check for panics
3. Checked the code for other potential panics

[tats-u]

Oh snap, looks like all of them are test code. Sorry to bother you.