Suggestion: gradual transition to TypeScript

[stackmystack]

I suggest transitioning the code base into TypeScript to reap all the benefits of:

1. static typing => catching bugs at compile-time, and
2. VSCode tooling which is superb for TS => refactoring, code-completion, etc.

I already made sure that the code I want to contribute for the Test Suite, and the code-base, is in ES Modules, which helps transitioning as well.

I am willing to do the effort by setting up all necessary tooling/config that allows gradual transition into TS.