Replaced the `shimmer_animation` third-party dependency with a custom, internal animation engine to improve package independence and maintainability.

- Introduced `AutoShimmerEffect`, a built-in animation engine utilizing `ShaderMask` and `LinearGradient`.
- Added `AutoShimmerDirection` enum to replace external shimmer direction types.
- Added `repeatDelay` support to `AutoShimmerAnimate`, `AutoShimmerConfig`, and related builders to control timing between animation loops.
- Deprecated the `interval` property in favor of `repeatDelay` for better naming consistency.
- Updated `ShimmerWrapper` to use the new internal `AutoShimmerEffect` instead of the external `Shimmer` widget.
- Updated unit tests to verify `AutoShimmerEffect` behavior, including `repeatDelay` and disabled states.
- Updated documentation and examples to reflect the new API changes and internal engine.
- Bumped package version to 0.0.2.