Proposal: replace qs dependency with query-string

[rapzo]

While there's nothing wrong with the qs dependency, it does a lot more than what is needed here and there are pleasant alternatives for simple purposes, such as query-string.
The qs module also requires the @types/qs dependency for typescript support where query-string does not (has its own).

It is already implemented in #205

The PR also comes with the fix for mocha's deprecation warning for using mocha.opts file, which would prevent updating to the next major. It's in a separate commit so it can be cherrypicked or even separated to another PR.

Thank you.

[JustinBeckwith]

Greetings! We're pretty happy with the qs module right now :) If there are specific things you need here, let us know and we'll consider it!