Expose internal colour distance feature (was: Detect whether an image is grayscale)

Hello, would you please consider adding such an operation?

No source file, test, or entry point is identified in the issue. Start by locating sharp's existing internal colour-distance or grayscale-related implementation, then determine the public operation and its expected behaviour. Done means the feature is exposed with documented, tested behaviour for detecting grayscale images or calculating colour distance.

Written by the indexing model from the issue text.