[relay-runtime] Add missing exports. (#38562)
[relay-runtime] Add missing exports. (#38562)

ReaderFragment is needed for types emitted by
relay-compiler-language-typescript
georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped