Add animations support

[juan-campuzano]

Animations

• Added the GoldenAnimationCapture extension on WidgetTester (lib/src/capture/golden_animation_capture.dart), enabling automated capture and combination of screenshots at defined animation steps, with support for layout configuration and timeline labels.
• Introduced the BcGoldenCapture.animation API, demonstrated in documentation and example tests, to facilitate animation golden tests with step-based frame capturing and configuration options. [1] [2]

Screenshot Capture Refactor:

• Refactored screenshot capture logic from the GoldenScreenshot class to WidgetTester extensions, simplifying usage and ensuring screenshots are tied to the test context. Methods such as captureGoldenScreenshot and combineGoldenScreenshots are now called directly on the tester. [1] [2]
• Updated example tests to use the new extension methods for screenshot capture and combination, replacing previous class-based usage. [1] [2] [3]

Documentation and API Updates:

• Expanded the README with a new section on animation testing, including usage examples and guidance for common animation scenarios.
• Updated library exports to include new animation-related configuration and capture files, and fixed logger export.