Add a `short2` attribute type, intended to store the existing
`CD_CUSTOMLOOPNORMAL` custom data type in order to move the custom
normal storage to generic attributes. This type probably won't get much
use besides that, but generally we don't have reasons not to add these
generic types, besides binary size. In the future we should consolidate
usage of `convert_to_static_type` to avoid large binary size increases
(about 700KB here) for this sort of addition.
This is the first step of #130484.
Pull Request: https://projects.blender.org/blender/blender/pulls/130530
a4c9b691a0