Remove XPlot.Plotly dependency and implement .NET Interactive-specific rendering as an extension

[jonsequitur]

We've had the goal to eventually unbundle XPlot.Plotly so that it's not an out-of-the-box dependency for dotnet-interactive, and people can install specific versions using #r nuget.

Extension support is stable enough now that the notebook-specific formatting can be added within the XPlot.Plotly NuGet package.