FragmentStore's _Data type argument does not accomodate @loading fragment's data shape

[gwennlbh]

Describe the bug

The current definition of FragmentStore enforces that _Data extends GraphQLObject:

export declare class FragmentStore<_Data extends GraphQLObject, _ReferenceType extends {}, _Input extends GraphQLVariables = GraphQLVariables> {

which is defined as follows:

export type GraphQLObject = {
    [key: string]: GraphQLValue;
};
export type GraphQLDefaultScalar = string | number | boolean;
export type GraphQLValue = GraphQLDefaultScalar | null | GraphQLObject | GraphQLValue[] | undefined;

however, @loading fragments' $data types are defined with a union to represent the loading state, that uses typeof PendingValue, but declare that they return FragmentStore instances.

This prevents library users from retrieveing the $data type given a FragmentStore instance type, somthing which is useufl to extend the library with generic utilities around loading states (i'm working on a Loading class that would allow .map-ing obejcts in potentially loading states for much better ergonomics, insteada of having to pass things to utility functions as I currently do)

See further discussion at https://discordapp.com/channels/1024421016405016718/1305501698244214794/1305501698244214794

Reproduction

https://stackblitz.com/edit/github-t4rqmk?file=src%2Flib%2Findex.ts&view=editor