Add `@metamask/rpc-methods` package

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.

Start by reviewing this repository’s existing package layout and the prior @metamask/rpc-methods package on npm, then check the prerequisite snaps issue. Done means a new @metamask/rpc-methods package provides a home for client-agnostic RPC method implementations, starts at major version 4 or greater, and includes a changelog explaining its history.

Written by the indexing model from the issue text.