[Feature Request] Implement `NumberFormat` part of `Stylesheet`

[omaus]

Is your feature request related to a problem? Please describe.
In OpenXml's Stylesheet class, things like CellFormat are stored. We need these to correctly set a cell we want to save as pure text as it otherwise leads to problems like in #19.

Describe the solution you'd like
Implement styles and map them from the Stylesheet object.

Edit: As discussed below, aim for NumberFormat first, then the rest at some later point.

[Freymaurer]

This might become critical in the future. Swate excel logic was formerly dependant on the numberFormat. A parameter which is currently not modeled via FsSpreadsheet.

For now i will set this issue to P1 and discussion. So its easier to keep track of!

[Freymaurer]

nfdi4plants/ARCtrl#81 <-

[omaus]

Important: Due to @Freymaurer 's mentioned urgency, we best aim for just implementing NumberFormat (aka CellFormat in MS Excel) since it is critically needed in ARCtrl.