Add support for Union types other than None

[nuclearglick]

Description

msgspec is wonderful, but only supports Unions that have None as the second type.

An example of the problem is:

import msgspec as ms
from typing import Union

class Example(ms.Struct):
    field1: Union[float, int, None] = 1.0

if __name__ == "__main__":
    temp = Example()
    encoded = ms.json.encode(temp)
    decoded = ms.convert(encoded, type=Example)

which will raise the error

TypeError: Type unions containing a custom type may not contain any additional types other than `None` - type `typing.Union[float, int, NoneType]` is not supported