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

**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](https://github.com/fslaborg/FsSpreadsheet/issues/21#issuecomment-2151684193), aim for NumberFormat first, then the rest at some later point.