refactor!: replace is-promise dependency with instanceof check

[jonkoops]

Replaces the is-promise dependency with an instanceof Promise check.

BREAKING: drops support for non-native promises

Closes #136

[jonkoops]

This PR should likely not be merged until the next major.

[jonkoops]

Just ping me whenever a new major is in the works and I will take a moment to rebase the work here.

[wesleytodd]

Really sorry to have left this one hanging for so long. You are right this is a breaking chance since it would drop support for some of the legacy promise forms that package supports. And looking back at the timing we had already cut the 2.x release line by the time this was opened anyway.

So, what can we do? I think the best option we could do is to check if it is anything other than a native Promise and print a deprecation warning in the 2.x branch. That would set us up to remove this package and fully drop support for anything but native promises in the 3.x line which will ship with express@6. Does that sound like a good next step to you?

[jonkoops]

Sure, I can go ahead and make a PR that logs a deprecation for this. Do you have any examples of how deprecations should be logged that I could use as inspiration?

[wesleytodd]

We have used depd for this in the past. This is what I could find in this repo, but we for sure used it in express as well.

[bjohansebas]

I like the idea of showing that warning

[jonkoops]

Created a PR to add the message under #154