feat(init): add `--init-private` config

[darcyclarke]

Summary

Add config for npm init that makes private configurable (defaults to --init-private=false)

Exit Criteria

• fully implemented
• tests are written for the config
• documentation is updated to specify functionality

[darcyclarke]

Example / Reference

Current

npm init 

package.json

{
  "private": "true"
}

# only applies to scoped packages today
npm publish --access=public

Expected

# --init-private=false
npm init

package.json

{}

# should be able to publish scoped packages publicly without --access=public
npm publish

[ljharb]

What's the opposite config? There should be a way to set it to either private or public, so that folks can insulate themselves fro a breaking change when the default changes.