feat: implement Vitest testing suite and GitHub Actions

[masonlet]

Implements test coverage across all modules (API, charts, rendering, utilities) with Vitest and integrates automated CI testing via GitHub actions workflow.

Changes

• Testing Framework: Integrated Vitest for unit testing and @vitest/coverage-v8 for coverage reporting
• CI/CD: Added a GitHub Actions workflow to run tests automatically on push and pull_request to main
• Refactoring: Minor improvements to parseIntSafe and nullish coalescing in params.js for better stability