make the license field of npm init more stubborn

[nlf]

Is your feature request related to a problem? Please describe.
Currently we set licenses to ISC by default

Describe the solution you'd like
We should not select a default license on behalf of our users. Instead we should default to omitting the license field entirely in non-interactive mode, and in interactive mode require that the user either provides a value or confirms that they do not want to select a license at all. As a bonus it would be nice if we had a site to link to that helps our users select a license that fits their needs.

> npm init
license: <enter>

leaving this field blank means your package is unlicensed. if you're sure, press enter again to continue
license: <enter>

Describe alternatives you've considered
The license default could change, but a new default means that we are once again providing a default that may not align with users needs. It feels best to not provide a license by default at all.

Additional context
npm/rfcs#597

[ljharb]

This is great - we should definitely include a link to help users choose.