feat: add CSSNumberish support with type-safe conversion helpers

[trurl-master]

🎯 Overview

This PR adds comprehensive support for the CSSNumberish type introduced in the Web Animations API, replacing the previous number type for timing properties.

✨ Key Changes

New Features

• CSSNumberish Helper Module: Centralized conversion utilities in cssNumberishHelpers.ts
• Type-Safe Conversions: Robust handling of CSSUnitValue (time units) and CSSMathValue
• Cross-Runner Compatibility: Enhanced runner abstraction with spyOn support
• Comprehensive Testing: Unit tests for helpers + integration tests across all runners

Technical Improvements

• Boundary-Layer Pattern: Convert CSSNumberish to number at API boundaries
• Zero any Types: Complete elimination of any in favor of unknown
• Graceful Degradation: Safe handling of unsupported CSS units with warnings
• Environment Awareness: JSDOM-compatible checks for browser APIs

🧪 Testing

All tests pass across:

• ✅ Jest
• ✅ Vitest
• ✅ SWC
• ✅ Examples

📦 Version

Bumps to 1.14.0 - new feature release

🔧 Breaking Changes

None - this is a backward-compatible enhancement that maintains existing API contracts while adding support for the new CSSNumberish type.