Add bounding boxes dataset

[awesomephant]

• When we build map apps we often needs bounding boxes for admin units, ie for zooming there in a robust way (see https://github.com/SWRdata/p141_artenvielfalt_feldvoegel/issues/19).
• Currently we have a bunch of hand-drawn bounds floating around, including in this repo: https://github.com/SWRdata/boundaries/blob/main/demo/src/lib/App.svelte#L51
• We could rationalise that by building bounding boxes for every admin boundary in the country here
• Mapbox ships this in their CSV files: https://github.com/SWRdata/boundaries/blob/main/demo/src/lib/App.svelte#L51