Add Moonbeam and Moonriver to supported chains

[eshaben]

Currently you can configure Moonbeam and Moonriver with the jsonRpcProvider, but it would be nice to add these chains as a part of the constant chains

[tmm]

Thanks for the feedback! We aren't adding new chains to the core right now until we figure out a scalable process. In the meantime, you can create your own chains by conforming to the Chain type.