Partition: calculate relative luminance for text

[rshen91]

Is your feature request related to a problem? Please describe.
Currently in pie charts there's logic in place for white text or black text

/** @internal */
export function colorIsDark(color: Color) {
  // fixme this assumes a white or very light background
  const rgba = stringToRGB(color);
  const { r, g, b, opacity } = rgba;
  const a = rgba.hasOwnProperty('opacity') ? opacity : 1;
  return r * 0.299 + g * 0.587 + b * 0.114 < a * 150;
}

but this needs to account for relative luminance to be in accordance with WCAG standards for text and background contrast.

Additional context
This logic may be in place already in this elastic/kibana#60261 and we can potentially just move the logic onto the elastic-charts end.

Kibana Cross Issues
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Checklist

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[markov00]

🎉 This issue has been resolved in version 19.5.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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