babel-relay-plugin: generalize warning about nodes fields

[josephsavona]

GraphQL servers may support various forms of pagination, for example:

type UserConnection {
  edges: [UserEdge] # a) access the relationship between the parent record and the User
  users: [User] # b) directly access the linked Users. 
}
type UserEdge {
  node: User
}

Relay currently supports only a) - the edges{node{...}} variant, and warns about case b) only if the field is called nodes. This PR changes the warning by detecting any use of the b) pattern regardless of the field's name.

cc @dschafer @leebyron @schrockn @yuzhi

[dschafer]

Might be worth expanding on this warning to explain why this is unsupported?

'`%s{...}` field on connection `%s` does not allow cursor insertion. ' +
'Use `edges{node{...}}` instead to allow Relay to auto-insert ' +
'`edges{cursor}` for efficient pagination',