Separate into core and express packages

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.

该 issue 未指定具体的文件或测试。首先检查项目和 package 的布局，然后检查 Chart 命名空间以及低级别的 TraceStyle 和 Chart.Scatter API，同时检查高级别的 Chart.Grid 和 Chart.Range 抽象。要视为完成，需要定义拆分为 core 和 express package，但所需的覆盖范围和依赖边界仍需明确。