Introducing densitymapbox traces

[etpinard]

Based #3988 (which is based off #3987)

Resolves #3110

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This PR adds a new mapbox-based trace type: densitymapbox

Example: https://codepen.io/etpinard/pen/KjZXOr

densitymapbox traces are essentially just a wrapper around mapbox-gl's "heatmap" layers - which we chose not the call heatmapmapbox as their algorithm is "too different" from our heatmap traces.

Please refer to the comments and descriptions in 7e15d81 for more info.

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cc @plotly/plotly_js @nicolaskruchten @chriddyp

[antoinerg]

It looks great 🎉 ! Nice job @etpinard 💪

💃

[meghnakayam]

This looks great. Is there a way to incorporate the same in python for Plotly Dash?

[nicolaskruchten]

@meghnakayam yep! This is coming to Plotly.py/Dash in a couple of weeks :)