[lockfile-explorer] Replace update-notifier with a built-in solution

[iclanton]

Summary

Removes the update-notifier npm dependency from @rushstack/lockfile-explorer and replaces it with a small built-in PackageUpdateChecker class.

The new implementation:

• Fetches the latest version from the npm registry using the global fetch API (Node 18+), with a 5-second abort timeout so it never hangs startup
• Caches results in ~/.rushstack/update-checks/<package>.json (consistent with the ~/.rushstack convention used by @rushstack/debug-certificate-manager)
• Starts the check concurrently with server setup and displays the notification inside app.listen, so it never blocks the server from starting
• Skips silently on network errors or cache write failures

Details

• apps/lockfile-explorer/src/utils/PackageUpdateChecker.ts — new self-contained utility (no new dependencies)
• apps/lockfile-explorer/src/utils/constants.ts — extracts LFX_PACKAGE_NAME / LFX_VERSION constants shared across the CLI entry points
• ExplorerCommandLineParser.ts — fires updateCheckPromise before server setup, awaits it in app.listen callback via printUpdateNotification()
• Removes update-notifier and @types/update-notifier from package.json
• Removes update-notifier from nonbrowser-approved-packages.json

Also includes: CLI help snapshot test and minor terminal/action cleanup.

How it was tested

Unit tests added for PackageUpdateChecker covering: fresh cache hit, stale cache re-fetch, missing cache, forceCheck, network error, non-ok HTTP response, isOutdated logic, wrong cacheVersion, custom cacheExpiryMs, and scoped package name sanitization.