Add hasDowngrades() export

[oalders]

Summary

Add a hasDowngrades(changes) function to lib/index.js that takes the output of diff() and returns true if any package was downgraded.

Motivation

The upcoming auto-merge-dependabot GitHub Action (oalders/auto-merge-dependabot#1) needs to determine whether a lockfile diff contains only upgrades. This logic belongs in diff-lockfiles rather than being reimplemented in the action.

Implementation

export function hasDowngrades(changes) {
  return Object.values(changes).some(
    ([oldVersion, newVersion]) =>
      oldVersion && newVersion && semver.lt(newVersion, oldVersion)
  );
}

• Returns true if any entry has a new version lower than the old version
• Skips added packages (oldVersion is null) and removed packages (newVersion is null)
• Should include tests for: no changes, all upgrades, mixed with downgrade, added/removed packages