v1.0.0 of spaced_repetition

Simple implementation of the SM-2 spaced repetition algorithm. Changelog excerpt: ### Added - Add the conventional package:spaced_repetition/spaced_repetition.dartentrypoint. - Add public Sm2and Sm2ResponseAPI types. - Add input validation for SM-2 quality scores and persisted review state. - Add a simple Dart usage example and an interactive Flutter example app. ### Changed - Convert the package from a Flutter package to a pure Dart package. - Align interval rounding and ease factor updates with the official SM-2 specification. - Update package metadata, README, API[...]