feat: Add NesIcons.palette

[Copilot]

Adds a palette icon to the NesIcons library using the specified 8×8 pixel sprite data.

Changes

• lib/src/widgets/nes_icon.dart — New palette entry in NesIconCollection:

  late final palette = NesIconData(
    MiniSprite.fromDataString(
      '8,8;1,-1;6,0;1,-1;2,0;1,1;2,0;1,1;11,0;1,1;4,0;1,1;11,0;1,1;3,0;1,-1;1,0;1,-1;7,0;2,-1;5,0;1,-1',
    ),
  );

• widgetbook/lib/widgetbook/use_cases/icons.dart — Added palette to the 8×8 icon showcase.
• CHANGELOG.md — Added entry under 0.31.0.

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