feat(xychart): add colorAccessor to AreaSeries

[vretamal]

🚀 Enhancements

• Add colorAccessor to XYChart AreaSeries prop that accepts a function that takes the dataKey and returns a color, replacing the default theme color

Based on #1489

[evansimons-chrono]

Hi @hshoff! Thanks for your work on visx.
I'm wondering whether minor patches / feature additions like this one are in a holding pattern for release under the v4 line? I don't spy a v3 release branch so imagining that's the case, but appreciate any insight.