Use $ReadOnlyArray type for connection data

[jaroslav-kubicek]

Hi good people 👋

I would like to suggest to use more strict type for connection data - $ReadOnlyArray<T>

Why?

We define all our types for data on GraphQL server as read-only, so passing data to connectionFromArray fails on flow unless we change the type to normal array or do workaround like [...children]:

connectionFromArray<Item>(children, args);
// Cannot call `connectionFromArray` with `children` bound to `data` because read-only array type [1] is incompatible with array type [2]

With these changes, it should be possible to opt-in for $ReadOnlyArray<T> while Array<T> would still work as well.

[kassens]

Thanks!