Add `Transfer` to `SupportedInterface`.

[q-uint]

Every DIP-721 compatible smart contract must implement this interface. All other interfaces are optional.
~ spec.md: DIP721 v2

The basic interface does/should not include the the transfer interface.

Expected

type SupportedInterface = variant {
  Transfer;
  Burn;
  Mint;
  Approval;
  TransactionHistory
};

Actual

type SupportedInterface = variant {
  Burn;
  Mint;
  Approval;
  TransactionHistory
};