Add new `Cell` mark

[techniq]

For 2 ordinal dimensions (ex. both as scaleBand) we should introduce a new Cell component. Can be used for heat maps and similar visualizations.

Examples

CleanShot.2025-04-06.at.14.09.32.mp4

CleanShot.2025-04-06.at.14.10.06.mp4

Related:

• Heatmap #13
• Bar Chart features / examples #21
• Add more Simplified charts #466

Reference

• https://observablehq.com/plot/marks/cell
• https://svelteplot.dev/marks/cell