Fix engine dependency version for Node versions > 11.1.0

[duncand]

Summary

Proposed change:

Fix engine dependency version for Node versions > 11.1.0;
now there is only a minimum Node dependency and not a maximum dependency;
the engine dependency is now a blacklist rather than a whitelist;
it indicates Node versions that are known NOT to work, and unknown future
versions are accepted by default rather than rejected by default.

This fixes a systematic problem where Nunjucks and all of its downstream
dependencies break every time a new Node version is released, despite the
Node update being backwards compatible, thus forcing a new Nunjucks
release such as 3.1.4 until the goalpost is soon moved again.

Closes #1168 .

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[Diokuz]

Guys, we need that thing)

[anru]

@fdintino pleeeease

[jcnventura]

Please merge!

[yogevyuval]

Let's merge this thing :)

[fdintino]

Thank you for this PR. I'll likely edit the changelog entry for brevity on release.