Add comprehensive test suite with Jest following obsidian-tasks testing approach

[Copilot]

This PR implements a comprehensive testing infrastructure for the Task Board plugin, following the approach used by the obsidian-tasks plugin as requested in the issue.

What's Added

Testing Framework:

• Jest with TypeScript support and ts-jest preset
• jsdom environment for DOM testing
• Proper mocking of Obsidian APIs to avoid runtime dependencies
• Test setup with custom matchers from @testing-library/jest-dom

Test Coverage (62 tests total):

• TaskRegularExpressions (25 tests): Task parsing regex patterns, checkboxes, hash tags, block links
• TaskItem Interfaces (14 tests): Data structure validation, priority mappings, JSON cache formats
• CheckBoxUtils (23 tests): Checkbox state switching, task validation, Obsidian settings

Infrastructure:

• Test scripts: npm test, npm run test:watch, npm run test:coverage
• Jest configuration optimized for Obsidian plugin development
• GitHub Actions integration - tests now run before releases
• Comprehensive documentation in tests/README.md

Example Test Output

PASS tests/utils/TaskRegularExpressions.test.ts
  TaskRegularExpressions
    ✓ should parse basic task
    ✓ should parse indented task  
    ✓ should match hash tags correctly
    ✓ should handle various checkbox states

PASS tests/utils/CheckBoxUtils.test.ts
  CheckBoxUtils
    ✓ should switch checkbox states correctly
    ✓ should detect completed tasks
    ✓ should validate task line format

Test Suites: 3 passed, 3 total
Tests: 62 passed, 62 total

Benefits

This testing infrastructure will significantly reduce manual testing time by automatically validating:

• Task parsing logic and regular expressions
• Checkbox state transitions and validation
• Data structure integrity and type safety
• Configuration handling and edge cases

The tests focus on core functionality that's difficult to manually test and prone to regressions, ensuring the plugin remains stable as new features are added.

Fixes #362.

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