Allow @stream on scalar lists

[AlicanC]

The @stream directive is currently only supported on lists of objects, but it should also be supported on lists of scalars.

Currently when @stream is used on a list of scalars, relay-compiler throws a InvalidStreamOnScalarField error.

So with a schema like this:

type Query {
  stringChunks: [String!]!
}

You can't do this:

query MyQuery {
  stringChunks @stream(initialCount: 1) # this could work but currently throws an error
}

So you have to modify your schema like this:

type WrappedString {
  value: String!
}

type Query {
  stringChunks: [WrappedString!]!
}

And do this:

query MyQuery {
  stringChunks @stream(initialCount: 1) {
    value
  }
}

This PR makes the necessary changes so @stream can be used on scalar lists.

Fixes #3755