[types][beta-5] fixed stacked scale, changed default type for data.labels

[xr0master]

fix #7821

Also changed the type for data.labels, the value can be string, Date, etc. Since not all types are known, set unknown by default.

[kurkle]

To me it looks like stack option is missing from IChartDataset, care to add that too?

As a side note, it looks like the documentation only mentions that for Bar, while it looks like its available to all chart types (https://codepen.io/kurkle/pen/bGejbNE).

[xr0master]

@kurkle I added stack to all dataset config.
I am not good at writing docs, but added dataset general config (Dataset Configuration) in the Data Structures

[kurkle]

I think we can come up with a better structure to the docs still, but also think its another subject.