feat(partitions): Small multiples legends

[monfera]

Summary

Flat legends (now sorted alphabetically; cursor hovering over Minerals - green - to highlight):

Hierarchical legends:

Checklist

Delete any items that are not applicable to this PR.

• This was checked for cross-browser compatibility
• Proper documentation or storybook story was added for features that require explanation or tutorials

[markov00]

I've few doubts on what was changed here:
https://github.com/elastic/elastic-charts/blob/9aa1d202b6ccc55dc3169f2a100a3b9ba3943761/integration/tests/__image_snapshots__/legend-stories-test-ts-legend-stories-should-have-the-same-order-as-nested-with-no-indent-even-if-there-are-repeated-labels-1-snap.png

I've added the legendStrategy to that story and it looks a bit wrong: it doesn't work correctly with all the strategies. Is that a wanted behaviour?

[codecov-io]

Codecov Report

Merging #1094 (d899405) into master (2a8bd89) will increase coverage by 0.00%.
The diff coverage is 43.75%.

@@           Coverage Diff           @@
##           master    #1094   +/-   ##
=======================================
  Coverage   71.71%   71.71%           
=======================================
  Files         383      381    -2     
  Lines       11924    11918    -6     
  Branches     2590     2598    +8     
=======================================
- Hits         8551     8547    -4     
+ Misses       3342     3340    -2     
  Partials       31       31           

Flag	Coverage Δ
unittests	71.71% <43.75%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...es/partition_chart/layout/types/viewmodel_types.ts	87.50% <ø> (ø)
.../partition_chart/layout/utils/highlighted_geoms.ts	19.04% <0.00%> (ø)
...types/partition_chart/renderer/dom/highlighter.tsx	15.55% <0.00%> (-0.54%)	⬇️
...partition_chart/renderer/dom/highlighter_hover.tsx	47.05% <0.00%> (-2.95%)	⬇️
...artition_chart/renderer/dom/highlighter_legend.tsx	44.44% <0.00%> (-8.89%)	⬇️
...on_chart/state/selectors/get_highlighted_shapes.ts	63.63% <0.00%> (ø)
src/specs/settings.tsx	90.32% <ø> (ø)
...chart_types/partition_chart/layout/utils/legend.ts	76.92% <68.75%> (-3.08%)	⬇️
...ypes/partition_chart/layout/viewmodel/viewmodel.ts	85.08% <100.00%> (-0.71%)	⬇️
...ypes/partition_chart/state/selectors/geometries.ts	76.97% <100.00%> (+2.37%)	⬆️
... and 3 more

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[markov00]

LGTM, agreed on the new partition flat legend alphabetical order.
Lens uses the flat version with only the root layer or the hierarchical version with all layers.

[nickofthyme]

🎉 This PR is included in version 27.0.0 🎉

The release is available on:

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

Your semantic-release bot 📦🚀