v2 changes

[nicolaskruchten]

Here's what we're going to do for v2, targeting a release in January or February 2020:

• switch to native-promise-only and officially drop support for IE9 and IE10
• stop exporting D3
• stop exporting Plotly.plot
• drop area, contourgl and pointcloud traces
• drop support for polar plots in scatter (aka "micropolar"?) i.e. scatter.r and scatter.t and layout.angularaxis and layout.radialaxis and anything else related (cc @alexcjohnson anything else I missed?)

For reference, the primary trigger for version 2.0 is the native-promise-only change, with which we fully drop support for IE 9/10, even though that support has been eroding severely over the last couple of years. This change is driven by a desire to remove function constructors from as many of our bundles as possible for security reasons, as tracked in #897. Since we're doing a v2, we're trying to shed as much deprecated stuff as we can without being otherwise too-disruptive.

[nicolaskruchten]

@alexcjohnson suggested the following, which I approve if they take less than a day:

• change isPlainObject - either just drop the second clause or use something like the new one I found in Validation fails when running via JsDom in NodeJs #5151 - in principle this needn’t be a major but it feels a bit risky to me for some straight JS users
• drop some other exports: it also may be nice to strip Plotly.Fx to just {hover, unhover, loneHover, loneUnhover} and Plotly.Plots to just {graphJson, resize}

[nicolaskruchten]

Note to self: the plotly.com/javascript docs will need to stop relying on the built-in D3 :)