Add transform components

[mhkeller]

I was thinking it could be useful to provide for various transform buckets that wrap <LayerCake> components and whose job it is to transform your data and then pass it back out. Some ideas

1. Stack To stack your data. It would return variables for data and flatData
2. Group For things like converting from wide to long data and grouping. Maybe WideToLong is a separate component
3. Facet This was the main use case that got me thinking. Similar to group but it creates small multiples. First attempt: https://github.com/mhkeller/layercake/blob/transforms/src/lib/transforms/Facet.svelte
4. Bin For easier histogramming

These should be able to be combined such as Facet and Bin could created a faceted histogram.