support init-type config

[nlf]

Is your feature request related to a problem? Please describe.
With the current trends of node developers tending to write es6 modules instead of commonjs ones, there is immediate value to be had by allowing users to set a default type for new modules when running npm init

Describe the solution you'd like
The init-type (or init.type) field should be supported. The default should be set to commonjs and the only other allowed value module.

Describe alternatives you've considered
Users could write their own init module, but documentation there is lacking and this field impacts all of the nodejs community in some way. It's best to support it as a first class value.

Additional context
npm/rfcs#347