[RRFC] `npm explain` should accept `--only`

[ljharb]

Motivation ("The Why")

I want to know if and how a dep foo came to be in my tree - but, i want to ignore dev deps.

Example

A transitive dep published a breaking change in a non-major, causing my tests for fail. I wanted to figure out whether this would break consumers, or, whether it was only breaking my tests.

How

Current Behaviour

I was able to simulate what I wanted with npm prune --production && npm explain foo, but then i had to reinstall after, which is annoying.

Desired Behaviour

npm explain foo --only=production should work.

(this should work in concert with #523)