refactor: move npm registry to datasource

lib/config/presets.js

lib/datasource/npm.js

lib/manager/npm/package.js

lib/workers/package-file/index.js

lib/workers/pr/changelog.js

lib/workers/repository/init/apis.js

test/config/presets.spec.js

test/manager/npm/package.spec.js

test/manager/npm/registry.spec.js

test/workers/pr/changelog.spec.js