Add environment variables using Belt CLI

[rakeshpetit]

In this PR, we add a new belt add env CLI command that sets up environment variable management in an Expo project using the EXPO_PUBLIC_ mechanism.

What the command does

• Installs dotenv as a dev dependency.
• Copies template files: .env.example, .env.test, jest.setup.env.js, and src/config/index.ts.
• Creates .env from .env.example if it doesn't already exist.
• Patches src/util/api/api.ts to replace the hardcoded GitHub API URL with EXPO_PUBLIC_API_BASE_URL.
• Patches jest.config.js to load jest.setup.env.js via setupFiles.

Refactoring existing notifications code

Reusable utilities were extracted as part of this work and applied to both env and notifications:

• src/util/patchFile.ts - generic read/search/replace/write function to patch existing project files.
• src/util/confirmToProceed.ts - interactive intro prompt shared across commands.
• handleCommitError moved to be used in multiple commands.

Both env and notifications command now have:

• Improved error handling using a try/catch with spinner.fail(error.message) so the terminal doesn't hang on errors.
• Multi-line user facing strings extracted to named constants for better readability of the code