Add nneoor to iset.mm
Add nneoor to iset.mm

The derivation of nneoor from nneo would rely on excluded middle
so we better provide nneoor as its own theorem. The proof is taken
from part of the previous nneo proof, and the nneo proof is now
much shorter as it can rely on nneoor .

metamath/set.mm,metamath/set.mm,metamath/set.mm