Fix node-sass mkdirp dependency

[jason-fox]

Proposed changes

Add missing mkdirp dependency to sass middleware. It looks like it has changed from callbacks to promises.

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• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

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