Add test suite for Big presentation library

[Copilot]

Closes: (issue reference needed)

Description

Added comprehensive test coverage for the Big presentation library using Jest and jsdom.

Test Infrastructure

• Jest with jsdom environment for DOM testing
• npm scripts: test, test:watch, test:coverage
• GitHub Actions workflow for CI on Node 18.x and 20.x
• CI/CD status badge in README
• .gitignore for node_modules and coverage artifacts

Test Coverage (17 passing tests)

• Utility functions: emptyNode(), ce(), parseHash() - validates core helpers
• API surface: Confirms window.big object, navigation methods (forward, reverse, go), and event listeners (keyboard, click, touch)
• Feature detection: Validates presentation modes (talk/print/jump), speaker notes, aspect ratio configuration
• File structure: Verifies big.js, big.css, and index.html exist with expected content

Integration Tests

8 integration tests skipped - jsdom doesn't support window.location.hash manipulation that big.js requires. Tests structured for future implementation with Puppeteer/Playwright.

Tradeoffs

• Skipped runtime integration tests due to jsdom limitations rather than adding Puppeteer dependency
• Used new Function() to load big.js in tests to avoid let re-declaration issues - acceptable for test-only usage

Example Usage

npm test                  # Run tests
npm run test:watch        # Watch mode
npm run test:coverage     # Generate coverage report

See TEST_README.md for detailed test documentation.

Original prompt

Create tests for this library

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[sroberts]

@copilot Add CI/CD status to the README.

[Copilot]

@copilot Add CI/CD status to the README.

Added CI/CD status badge to the README showing the Tests workflow status. (752677a)