feat: make FileType(s) public

[alkasm]

Move FileType and FileTypes from nominal._utils to nominal.core. To preserve backcompat for a time, a custom module __getattr__ provides the types at the old path with a deprecation warning:

>>> from nominal._utils import FileTypes
<stdin>:1: UserWarning: nominal._utils.FileTypes is deprecated and will be removed in a future version, use nominal.core.FileTypes instead.
>>> from nominal._utils import FileType
<stdin>:1: UserWarning: nominal._utils.FileType is deprecated and will be removed in a future version, use nominal.core.FileType instead.