Fixes #3185 - Invalidate less-node file cache if modified - #3219

Cool! When u think we can expect less 3.0.5 (or a higher version) including this fix :)?

@hawkerboy7 I'd like #3228 reviewed and merged before I publish a new release, because it improves upon a new parsing feature previously merged this week.

@hawkerboy7 I realized I could publish this as a beta. So you can get this merged-in fix if you install with npm install less@beta.

@matthew-dean Ahh very nice, thank you! =)

@hawkerboy7 You're very welcome! 😁

Could the asynchronous code path here use fs.stat instead of fs.statSync?

Synchronous calls stall the entire Node.js process, which I've measured can cause significant performance impacts in e.g. build processes.

@akx Yes, it could... it's just callback hell to write. If Less were updated to ES7/Babel-transformed code, it would be a simple matter of changing to await.

Part of the challenge with that bit of code is that there is a feature for sync imports (I guess to make it easier for building tests?). So I was trying to avoid duplicated code by essentially having the less-node file adapter either call a Promise or mock a promise and call things synchronously.