✨ statically embed nep-50 in the numeric scalar types (#485)

* ✨ statically embed nep-50 in the numeric scalar types

* ✨ add `_numtype.CanPromote` as interface to the nep-50 scalar promotion rules

* ♻️ rename `CanPromote` to `CanCast` and `__promote__` to `__nep50__`

* 🚚 rename nep-50 test

Author: jorenham