Color legends should display scale unknown & extremes when applicable

[Fil]

The color legends should display the unknown color if it is set, possibly with an unknownLabel that default to "unknown" (or "N/A", "invalid"?)

Note that if d3.scale adds scale.high / scale.low (or under/over, or extremes…) #258, these parameters should also be shown in the legends if they are set.

H/T @ananya-roy

[Fil]

An example where it would be useful: https://observablehq.observablehq.cloud/framework-example-loader-duckdb/