Separate into core and express packages

[kMutagene]

Just as the python lib, Plotly.NET provides both low-level access to plotly.js figure generation (e.g.TraceStyle functions, Chart.Scatter) and high-level abstractions for composite charts (e.g. Chart.Grid, Chart.Range).

Once a full low-level coverage is reached on 2.x.x, it might be beneficial to split these layers to both make the chart namespace more concise and add dependencies such as a data frame library to the high level abstractions.