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

https://github.com/user-attachments/assets/fa1cab06-79f3-4f46-8237-58eed3775dc2

https://github.com/user-attachments/assets/3889d302-a144-4983-a8df-3952d984fd82

Related:

• #13
• #21
• #466

Reference

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