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.

Meshes get a edge_creases RNA property like the existing vertex
property, to provide more efficient access to the data in Cycles.

One test failure is expected, where different rounding between float
the old char storage means that 5 additional points are scattered in
a geometry nodes test.