Add a top-level Makefile in js-sdk to simplify running tests and linters

Currently, contributors to the JS SDK have to manually use npm commands (e.g. npm test, audit npm run lint) or check the CI workflow to figure out how to run tests and lint locally.
This adds friction for contributors, especially when switching between SDKs in different languages.

A standardized Makefile would make it much easier to test and lint the code locally with simple, consistent commands.