Charts: Add custom legend rendering

[adamwoodnz]

Fixes CHARTS-119: Pie/Donut/Semi circle charts: Support custom rendering of legend, allowing rendering of a legend like this, which uses CSS grid for layout and has conditional content (the comparison %). The custom legend has access to the colors it should use to match the chart.

Proposed changes:

• Adds support for rendering a custom legend
• The render prop receives the generated legend items for the chart, so the consumer can extract whatever data is needed, and return any markup.

Other information:

• Have you written new tests for your changes, if applicable?
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Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to http://localhost:50240/?path=/story/js-packages-charts-types-donut-chart--custom-legend&args=themeName:woo
• Switch theme and the legend item colors should update along with the chart
• Check other Pie and Donut chart legend stories and ensure the legend is not obscured

[github-actions]

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bin/jetpack-downloader test jetpack charts-119-piedonutsemi-circle-charts-support-custom-rendering-of

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[github-actions]

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[adamwoodnz]

Extract the color from the legend items in the global charts context, which have a resolved color based on the theme, group, overrides, etc.

[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/js-packages/charts/src/stories/sample-data/index.ts	20/20 (100.00%)	0.00%	0 💚

Full summary · PHP report · JS report

[adamwoodnz]

Swap this order to match that in Woo Analytics