Setup CI workflow and testing environment

[pniedzwiedzinski]

This PR introduces a Continuous Integration (CI) workflow using GitHub Actions to ensure the project builds and tests pass on every push and pull request to the main branch.

It also sets up a testing environment using Jest and ts-jest, including:

• Unit Tests: For utility functions in src/lib/utils.ts (sanitize, proposeEmail, parseFormUrlEncoded).
• Integration Tests: An example test for onFormSubmit in src/handlers/formHandler.ts that demonstrates how to mock Google Apps Script global objects (SpreadsheetApp, GmailApp, etc.) locally.
• Documentation: A docs/TESTING.md file explaining how to run tests and write new integration tests with mocks.

This addresses the requirement to verify compilation on PRs and provides a foundation for robust testing of the Google Apps Script project.

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PR created automatically by Jules for task 3485398679198402727 started by @pniedzwiedzinski

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