feat: add DensityMap trace for the MapLibre map subplot #231

Job	Run time
Build Wasm Examples (callback-example)	1m 2s
Build Wasm Examples (basic)	51s
Rustfmt	13s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 29s
Build Book	3m 15s
Code Coverage	15s
Clippy	3m 3s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 17s
semver	44s
Build Examples (images)	1m 1s
Build Examples (customization/consistent_static_format_export)	1m 45s
Build Examples (scientific_charts)	25s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 16s
Build Examples (basic_charts)	38s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	3m 7s
Build Examples (subplots)	33s
Build Examples (kaleido)	1m 25s
Build Examples (customization/density_mapbox_example)	32s
Build Examples (statistical_charts)	27s
Build Examples (themes)	35s
Build Examples (ndarray)	27s
Build Examples (3d_charts)	35s
Build Examples (custom_controls)	37s
Build Examples (customization/multiple_plots_example)	30s
Build Examples (financial_charts)	31s
Build Examples (shapes)	29s
Build Examples (maps)	52s
	33m 54s

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