npm engine requirement isn't taken into account at install time

[minrk]

In #323, we bumped the minimum node version, in part because I had assumed npm's node engine resolution worked like pip's (find the latest version we can run), but it doesn't. Instead, it just warns that it won't work. It appears the best we can hope for is turning that warning into an error by adding strict engine checks, it cannot be considered an input to the installation request (which seems wild to me!).

Since the latest Ubuntu LTS (20.04) is still on node 10, I think we should bump minimum node back down to 10, which means dropping commander back to 7.x.

At the very least, in the future we should consider bumping the engine a major revision (which @consideRatio originally proposed, but I recommended against due to this misunderstanding), to make it easier to communicate the boundaries.