Add isMinter function to the IERC721Minter interface

[sebastianst]

We should extend the IERC721Minter interface to

interface IERC721Minter {
    function mint(address to, uint256 id) external;
    function isMinter(address maybeMinter)  public view returns (bool);

}

so that dApps can check whether the Erdstall ERC721MinterHolder can mint on it. A dApp would just call isMinter with the holder address and it would return true if it's an ERC721Minter and the holder can mint. If the call errors, it means that the contract doesn't implement isMinter, so in particular, it's then not a ERC721Minter.