feat(legacy-plugin-chart-calendar): increase the contrast of calendar heatmap color and label

[stephenLYZ]

🏆 Enhancements
The algorithm is to decide label color in white or black depending on background color (refer to https://stackoverflow.com/questions/3942878/how-to-decide-font-color-in-white-or-black-depending-on-background-color)

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[rusackas]

This is super nifty! I wonder if we should label it as something more descriptive, like "getContrastingColor" so that we can eventually (after monorepo migration, perhaps) export it from a shared module, like Superset's colorUtils.ts file.

[codecov]

Codecov Report

Merging #1452 (ca7d2aa) into master (4262d4a) will increase coverage by 0.14%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #1452      +/-   ##
==========================================
+ Coverage   30.48%   30.62%   +0.14%     
==========================================
  Files         498      499       +1     
  Lines       10019    10048      +29     
  Branches     1691     1699       +8     
==========================================
+ Hits         3054     3077      +23     
- Misses       6719     6723       +4     
- Partials      246      248       +2     

Impacted Files	Coverage Δ
packages/superset-ui-core/src/color/index.ts	100.00% <ø> (ø)
...cy-plugin-chart-calendar/src/vendor/cal-heatmap.js	0.00% <0.00%> (ø)
packages/superset-ui-core/src/color/utils.ts	100.00% <100.00%> (ø)
...lugin-chart-echarts/src/Timeseries/transformers.ts	42.24% <0.00%> (-1.13%)	⬇️
...uperset-ui-core/src/query/types/AnnotationLayer.ts	100.00% <0.00%> (ø)

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[zhaoyongjie]

LGTM, Thanks for adding the complete testing.