Add magic comment to omit a code section from transpiler output

[frangio]

// oz-transpiler omit-begin
...
// oz-transpiler omit-end

Would be used for e.g. EIP712:

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/446cd9f5fb5d982863ffaecf5dd40489f595d7f6/contracts/utils/cryptography/EIP712.sol#L31-L33