Add ESLint typescript configuration and address warnings in src folder

[oleiade]

This Pull Request adds a ESLint configuration to the src folder, specific to typescript, based on @EdvinasDaugirdas 's recommendation to use the eslint-typescript:recommended plugin.

This PR also addresses linter warnings in the src/ folder, and adds a linting Github action.

[codebien]

It would be great if we could have a shared linter config as a single source of truth and import it here and in the docs. I could stay under our k6/examples folder.

[codebien]

wow 🤕

[oleiade]

Yup, I'm sure there's something more clever to do there, but in the meantime, it will do.

What I learned in the process is that the unknown type is less permissive than any and will force you to do type assertions whenever you're using that value. See this article that I found useful.

[oleiade]

I'm much more concerned about the current inability to make type checking on the crypto.hmac calls though to be fair 👍🏻

(looking into switching to webcrypto there down the road)

[oleiade]

It would be great if we could have a shared linter config as a single source of truth and import it here and in the docs.
I agree. I'm just not sure how we would set and communicate the source of truth, though 🤔 In a convenient way, I mean.

Also, in some recent discussions, we questioned whether the examples were something we wanted to keep or care about, so I'm not sure this would be the right place, in light of that.