Handle complex expressions in `add()` & `subtract()`

Created by: ffoodd

Fixes #33953 (closed)

I don't expect any regression, but who knows :D Need to consider about backporting this to v4, if this gets merged.

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SassMeister demoing this: https://www.sassmeister.com/gist/daceb834e14e092919b5c6ebaedf317b