Move `mongodb` to peerDependencies

[rtritto]

mongodb should be moved from dependencies to peerDependencies.

Context

npm 6 doesn't autoinstall peerDependencies.
npm 6 is used in Node 14 that is deprecated.
npm versions > 6 will autoinstall peerDependencies.

[hasezoey]

why should mongodb be moved to peerDependencies?
any modern package manager will automatically de-duplicate a package if possible and mongodb-memory-server-core does not export any mongodb variables, it is also tested against that version and will not interfere with other installed mongodb versions installed

[rtritto]

By this way, can be used a specific version and/or same version in dependencies.