Fix private method access issue in AnimationEffect for transpiled environments

[trurl-master]

Fixes #62

Problem

When using mockAnimationsApi(), users encounter a TypeError: Cannot access private method error. The error occurs in getComputedTiming() which calls #getNormalizedDuration() from the MockedAnimationEffect class.

Root Cause

The issue stems from private method inheritance limitations in certain JavaScript environments:

1. Class Structure: MockedKeyframeEffect extends MockedAnimationEffect
2. Private Method Access: getComputedTiming() in the parent class calls the private method #getNormalizedDuration()
3. Environment Dependency: While this works in modern Node.js environments, some transpiled/bundled JavaScript environments have stricter private method access rules that prevent child classes from accessing parent private methods, even through inheritance

Solution

Convert the private method #getNormalizedDuration() to a protected method _getNormalizedDuration():

// Before (causing issues)
#getNormalizedDuration(): number { ... }

// After (compatible with all environments)  
protected _getNormalizedDuration(): number { ... }

Benefits

• ✅ Maintains encapsulation (still not public)
• ✅ Allows child classes to access the method in all JavaScript environments
• ✅ Preserves existing functionality
• ✅ All tests continue to pass
• ✅ No breaking changes to the public API

Changes

• src/mocks/web-animations-api/AnimationEffect.ts: Convert private method to protected
• package.json: Bump version to 1.15.1

Testing

• ✅ Linting: No ESLint errors
• ✅ Type Checking: No TypeScript errors
• ✅ All Tests: 113 tests passed across all test suites (Jest, Vitest, SWC)

This fix ensures compatibility across different JavaScript transpilation environments while maintaining the intended class hierarchy and functionality.