feat: Add {x,y}gap parameter to heatmaps #150

Job	Run time
Rustfmt	16s
Build Book	3m 43s
Build Wasm Examples (basic)	54s
semver	34s
Clippy	4m 27s
Code Coverage	9s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	4m 42s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	2m 30s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	2m 39s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	5m 36s
Build Wasm Examples (callback-example)	47s
Build Examples (basic_charts)	36s
Build Examples (customization/multiple_plots_example)	32s
Build Examples (scientific_charts)	32s
Build Examples (kaleido)	1m 29s
Build Examples (3d_charts)	35s
Build Examples (customization/density_mapbox_example)	30s
Build Examples (shapes)	29s
Build Examples (customization/consistent_static_format_export)	1m 37s
Build Examples (custom_controls)	33s
Build Examples (subplots)	36s
Build Examples (ndarray)	32s
Build Examples (financial_charts)	35s
Build Examples (statistical_charts)	34s
Build Examples (maps)	36s
Build Examples (themes)	28s
Build Examples (images)	52s
	37m 23s

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