feat: npm shrinkwrap

lib/manager/npm/extract.js

lib/manager/npm/package.js

lib/manager/npm/resolve.js

lib/platform/github/index.js

lib/workers/branch/lock-files.js

lib/workers/branch/npm.js

lib/workers/package-file/index.js

test/manager/__snapshots__/resolve.spec.js.snap

test/manager/resolve.spec.js

test/workers/branch/__snapshots__/lock-files.spec.js.snap

test/workers/branch/lock-files.spec.js

test/workers/branch/npm.spec.js

test/workers/package-file/index.spec.js