swift-fsrs improvements

[bootuz]

Since my app depends on this library and the lib is a bit outdated, I'd like to contribute a few improvements to swift-fsrs. Here's the roadmap I have in mind:

1. FSRS v6 implementation — first PR is ready for review.
2. Swift Concurrency support — make the package Sendable-correct and async-friendly.
3. Migrate tests to Swift Testing — replace XCTest with the new testing framework.
4. Optimizer — add it via fsrs-rs bindings, following the approach used by other FSRS libraries.

Happy to discuss any of these or split them into separate issues if preferred.

[aehlke]

Thank you for adding the optimizer in particular!

[bootuz]

@aehlke I hope it won't take forever to accept my PRs :(

[L-M-Sherlock]

@bootuz we need active contributors and maintainers. Would you like to join OSR and maintain swift-fsrs?

[bootuz]

@L-M-Sherlock I'd love to! 😱

[L-M-Sherlock]

The main branch is protected so you need make PR for each contributions.

And ts-fsrs is a good reference. If you have anything unsure in the design of API, you can ask the main maintainer of ts-fsrs.

[bootuz]

Concurrency updates has arrived: #33

[ningkaiqiang]

Thanks, @bootuz, amazing improvements plan!!

[bootuz]

Swift Testing migration: #32