ensure TypeScript definitions are sound during testing

[glynnbird]

I noticed the TypeScript definitions weren't syntactically correct, so I fixed them and ensured that the npm run test command checks the TypeScript syntax too.

Overview

Fix TypeScript definition and ensure that the tests can't pass with invalid TypeScript definitions in the future.

Testing recommendations

npm run test

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;