Add dice background grid (planned for next release)

[maflot]

I’ll be adding an optional dice background grid to ggdiceplot in the next release.

This is based on recent work on the DicePlot implementation in kuva (see: Psy-Fer/kuva#55), where introducing a grid aligned with the canonical 3×3 die layout turned out to significantly improve visual structure.

Rationale

• Provides clearer spatial structure for each die cell
• Helps maintain readability as the number of categories increases (e.g. up to 9)
• Works well in combination with the updated legend design

Planned behavior

• Optional grid rendered behind each die
• Aligned with the standard die-face layout
• Minimal visual weight (subtle lines, low alpha)

More details and API specifics will follow with the implementation.