[v4] Roadmap #282

This issue acts as a roadmap aggregating the features exclusively for the next version of this library.

Development of Plotly.NET 3.0 will be conducted on the v3 branch

The following features are most likely not backwards compatible and cannot be added as feature versions for v2.

• migrate to immutable dynamic object - this makes all charts immutable, removing many confusing problems coming from mutation of charts) (Migrate to immutable dynamic object #176)
• Separate into core and express package - Currently, there is a mix of low and high level APIs combined in a single package - Plotly.NET. In the future, especially when adding more and more high level APIs - it is beneficial to separate these in two packages, just as the python package did. (Separate into core and express packages #209)