Add `@metamask/rpc-methods` package

[rekmarks]

Once https://github.com/MetaMask/snaps/issues/1860 is closed, we should add a new package @metamask/rpc-methods to this repository. This is so that we can establish a home for client-agnostic RPC method implementations. This package will be a continuation of https://www.npmjs.com/package/@metamask/rpc-methods and will have to begin at major version 4 or greater. The changelog should explain its history.