Add greeter.ts with greet function

[dzianisv]

Summary

This PR introduces a new greeter.ts file containing a simple greet function that returns a greeting message for the given name.

Checklist

• Added greeter.ts file with the greet function.
• Type-checking (npm run typecheck) passed successfully.
• CI checks pass.
• Local tests pass.

Verification

Run npm test and ensured all tests pass.