test and refactor lib/npm.js and bin/npm-cli.js

[isaacs]

• Remove some affordances that are no longer relevant (WScript messaging, onload-script support, npm.commandName instead of npm.commands['command-name'](), optional callback/args lists, etc.)
• unit test up to 100% coverage
• modernize code (make npm an instance of a class, const/let/arrow-functions, organize and clean up, etc.)

[isaacs]

Also going to update help.js in this, since the changes to the npm object have a pretty significant impact there.

[isaacs]

Note: this should land after the lib/ls.js refactor, and remove the REMOVEME commit.

[ashward]

Apologies for jumping on an old issue, but I wondered why onload-script support had been removed, and whether there is another way to do achieve a similar thing (hooking into the npm script whenever it's invoked) on a per-repository basis?

The reason I ask is that my security wrapper relies on this functionality, and as it stands this doesn't work with npm 7: https://github.com/ashward/jsinstallguard