Update algorithm to FSRS 4.5

[SplittyDev]

Hey, this PR updates the algorithm to FSRS 4.5.

Changes

• Introduced decay and factor params
• Updated forgetting curve calculation
• Simplified some of the code for clarity and deduplication
• Fixed tests to pass again

Sources

New values for the forgetting curve etc. have been taken from:

• https://github.com/open-spaced-repetition/fsrs4anki/wiki/The-Algorithm#fsrs-45
• https://github.com/open-spaced-repetition/go-fsrs/blob/main/params.go
• https://github.com/open-spaced-repetition/go-fsrs/blob/main/fsrs.go

Further, the tests have been updates to use the same values (for date and weights) as the Go tests:

• https://github.com/open-spaced-repetition/go-fsrs/blob/main/fsrs_test.go

This should make it easier to maintain both versions in the future and test them against each other.

Misc

Resolves #10

[SplittyDev]

@ishiko732 I think I've addressed all of your concerns now. Please let me know if the 0 handling for forgettingCurve(...) where stability == 0 is fine.

[ishiko732]

@L-M-Sherlock @bensmiley cc