flags aren't supported in decode_vyper_object

[cyberthirst]

pip show boa: 0.2.5
vyper --version: 0.4.0+commit.e9db8d9
python --version: 3.11.10

import boa

def test():
    code = """

flag S:
    Admin
    User

s: S

@external
def foo():
    pass
    """

    c = boa.loads(code)
    c.foo()
    storage = c._storage.dump()
    print(storage)

test()

outputs:

<storage: s=unimplemented decoder for `flag S('Admin','User')`>

more generally, any consumer of decode_vyper_objec would fail to decode the flag