Move `CD_CUSTOMLOOPNORMAL` to the newly added
`CD_PROP_INT16_2D` generic attribute type. This is similar to
previous commits moving specific custom data types.
The attribute name is `custom_normal`. When the attribute with
that name is on the face corner domain, the code will interpret it
as stored in the existing deformation-invariant spherical coordinate
space.
The API remains the same, with the additional opportunity to edit
custom normal data as an attribute directly (which admittedly is fairly
unintuitive currently).
See #130484.
Pull Request: https://projects.blender.org/blender/blender/pulls/130689
f9b627d29c