feat: Enhance ModFill coloration

[gwhitney]

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• Consolidates ModFill color parameters into a single formula. This removes the issue about highlighting persisting when it should not.
• Changes Sunzi mode to a boolean parameter, since the opacity can now just be encoded in the background color.
• Updates documentation.

Resolves #519.

Will need CI snapshot update because parameter tab layout changed slightly.

[gwhitney]

One question for @katestange to consider in review: All of the Featured Specimens using modfill are either monochrome or two-tone, just because of the former structure of the color options. Is it worth seeking an example where use of a more extensive palette in ModFill is interesting/helpful, and adding that to the Featured Specimens or subbing it in for an existing one? One simple version of this would be to just change the Fill color for "Woven Residues" to be rainbow(n/2) rather than the default black, since it's just for visual effect anyway... Otherwise, I think all is well here, but of course look forward to other review feedback if any.

[katestange]

This all looks good and everything seems to work. And yes, I like rainbow(n/2) if you'd like to do that before I merge.

[katestange]

CI needs update, then I'll merge

[gwhitney]

OK, all should be well now.