feat: pin npm dependencies using lock files

When package-lock.json or yarn.lock files are present, and Renovate needs to “pin” any dependencies (e.g. after onboarding), it will use whatever version is in the lockfile rather than the latest version on npm js that satisfies the semver range. This should increase the chance that Pin Dependencies PRs pass tests, as it should use as close as possible dependencies as the previous time the lock file was committed to master. Thanks to @alexeagle for first suggesting this.

Closes #1362