feat: Add Jest and Testing Library setup for MERN templates

[amanvatss]

This pull request introduces a pre-configured testing environment for the MERN templates (both JavaScript and TypeScript), enabling developers to run tests out-of-the-box with npm test.

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Before This PR

• The generated templates had no built-in testing capabilities.
• Developers had to manually install, configure, and debug a testing framework from scratch.

After This PR

• Both the JavaScript and TypeScript MERN templates now include a fully configured Jest and React Testing Library setup.
• Developers can immediately start writing tests for their components.
• A sample test (App.test.js/.tsx) is included to verify the setup and provide an example.

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Changes Implemented

• Dependencies Added: jest, jest-environment-jsdom, @testing-library/react, @testing-library/jest-dom.
• JavaScript Template:
  • Configured Babel (.babelrc) for JSX transformation.
  • Added jest.config.js, jest.setup.js, and a fileMock.js for static assets.
• TypeScript Template:
  • Added ts-jest, @types/jest, and typescript dependencies.
  • Created a custom jest.config.js to handle TypeScript and JSX compilation correctly for the test environment.
• Added npm test script to the package.json of both client templates.

This feature significantly improves the quality and completeness of the scaffolded projects, promoting best practices from the start.

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Proof of Work (Test Results)

Here are the screenshots showing the tests passing for both the JavaScript and TypeScript templates.

JavaScript MERN Template:

TypeScript MERN Template:

Closes #111