feat: add DensityMap trace for the MapLibre map subplot #222

Job	Run time
Code Coverage	16s
semver	58s
Build Book	5s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	7m 55s
Rustfmt	15s
Clippy	4m 0s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 20s
Build Examples (customization/multiple_plots_example)	36s
Build Examples (themes)	38s
Build Examples (maps)	45s
Build Examples (ndarray)	34s
Build Examples (3d_charts)	37s
Build Examples (customization/density_mapbox_example)	37s
Build Examples (kaleido)	1m 21s
Build Examples (financial_charts)	35s
Build Examples (statistical_charts)	35s
Build Examples (images)	51s
Build Examples (scientific_charts)	33s
Build Examples (basic_charts)	35s
Build Examples (custom_controls)	35s
Build Examples (customization/consistent_static_format_export)	1m 45s
Build Examples (shapes)	34s
Build Wasm Examples (callback-example)	1m 0s
Build Wasm Examples (basic)	1m 2s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 28s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 15s
Build Examples (subplots)	33s
	37m 18s

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