Add support for `typing.TypeAliasType`

[wikiped]

Description

Currently (msgspec version 0.18.4, python 3.12) the following:

import msgspec as ms
from typing import Annotated

NonEmptyStringAlias = Annotated[str, ms.Meta(min_length=1)]
type NonEmptyStringTypeAlias = Annotated[str, ms.Meta(min_length=1)]

class A(ms.Struct):
    name: NonEmptyStringAlias

class T(ms.Struct):
    name: NonEmptyStringTypeAlias

print(ms.json.decode(ms.json.encode(A('test')), type=A))

print(ms.json.decode(ms.json.encode(T('test')), type=T))

Will fail on the last line:

A(name='test')
...
TypeError: Type 'NonEmptyStringTypeAlias' is not supported

Considering that:

print(type(NonEmptyStringTypeAlias))
# typing.TypeAliasType

print(type(NonEmptyStringAlias))
# typing._AnnotatedAlias

NonEmptyStringTypeAlias.__value__ is NonEmptyStringAlias
# True

Would it make sense to add support for the typing.TypeAliasType which is created with type statements (since python 3.12)?

[jcrist]

Sure, this is definitely in scope. Adding support for TypeAliasType instances should be fairly straightforward, with the exception of recursive alias definitions (these would require a major refactor of msgspec's internals).