Allow comma-separated, multi-interface `implements`

[pcaversaccio]

See #3643 (comment).

from ethereum.ercs import IERC20, IERC20Detailed
from snekmate.tokens.interfaces import IERC20Permit
implements: (
    IERC20,
    IERC20Detailed,
    IERC20Permit,
)

Follow-up to PR: #4762.