This is very similar to D14077. There are two differences though.
First is that vertex creases are already stored in a separate layer, and
second is that we can now completely remove use of `Mesh.cd_flag`,
since that information is now inherent to whether the layers exist.
There are two functional differences here:
- Operators are used to add and remove the layer instead of a property.
- The "crease" attribute can be created and removed by geometry nodes.
The second thing should make various geometry nodes a bit faster,
since the "crease" attribute was always processed. Crease is now
interpolated generically in the CustomData API too, which should
help maintain the values across edits better.