Integrate Jest alongside Node.js tests

[AvaneeshKesavan]

This PR adds optional Jest support to the Stringzy project while keeping the existing Node.js native test runner intact.

Changes included:

• Added jest.config.ts for Jest configuration.
• Updated package.json scripts to allow running tests with Jest (npm run test:jest) alongside Node.js tests (npm run test:node).
• Ensures all existing tests run successfully under Jest.
• No changes were made to the source code or existing functionality; this only enhances the testing workflow.

So you can now choose between the Node.js test runner and Jest.