RFC: disallow non-root node(id:...) fields

[josephsavona]

Despite the Object Identification documentation, there has been some confusion in the community about whether the node field should be at the root or nested inside Viewer. This adds a validation check to the babel relay plugin to reject non-root node(id: ...) fields.

[wincent]

This is probably worth it, to avoid confusion.

I guess if I ever wanted to re-use node(id: $id) as field name deep down in the tree (probably a terrible idea) I'd have to come up with another name, or at least a different/additional argument, which seems fair.

[josephsavona]

@facebook-github-bot import

[facebook-github-bot]

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