Add npm check to doctor

[shadowspawn]

Problem

People may have installed npm separately before trying n for managing Node.js (node + npm) and shadow the version installed by n.

See: #762

Solution

Add a check to n doctor. This could be detected at install time, but just adding to n doctor for now.

Generates warning like:

Checking npm install destination...
There is an active version of npm shadowing the version installed by n. Check what is in PATH (unless this is expected).
   installed : /usr/local/bin/npm
      active : /Users/john/my_npm/bin/npm

ChangeLog

• check for possible problem with multiple npm locations when running n doctor

[shadowspawn]

Tracking new cases where an install time check might have helped:

• Installing v12.22.12 does not install the correct npm version #765 (HomeBrew)