refactor: replace console.warn with process.emitWarning

[lamweili]

To have consistent warnings from #953 (comment) by @JPeer264

Yes process.emitWarning sounds also like a great option to add (just my opinion on it). However, a separate PR would be nice if you want to go the extra mile as we squash our PRs. @manidlou @RyanZim @jprichardson any objections?

Prints out via process.emitWarning in the following format:

[RyanZim]

Should we consider this: https://nodejs.org/api/process.html#avoiding-duplicate-warnings?

[lamweili]

My motivation was to retain the behavior of the console.warn which warns duplicatedly if called multiple times.

Should we change it to use https://nodejs.org/api/process.html#avoiding-duplicate-warnings instead, so it only warns on the first time?

What do you guy think? Let me know and I'll make the adjustments. 😊
@RyanZim @JPeer264 @manidlou @jprichardson

[JPeer264]

I am fine with either. Thanks for the changes and your efforts :)