fix(core): Fix legacy GCG generated Variables types

[kitten]

Resolves #3026

Summary

Fix generated empty Variables type as passed to generics, that outputs a type of { [var: string]: never; }.
A legacy/unsupported version of typescript-urql, which wraps urql's React hooks, generates
empty variables types as the following code snippet, which is not detected:

type Exact<T extends { [key: string]: unknown }> = { [K in keyof T]: T[K] };
type Variables = Exact<{ [key: string]: never }>;

NOTE: We cannot guarantee that this actually fixes @graphql-codegen/typescript-urql, which will going forward be unsupported. Generally, we choose not to create test harnesses for outside type generators like these, until we'd have our own type/code generator.

We cannot guarantee that types of hook/wrapper generators don't break, and instead recommend and support type/code generators that output TypedDocumentNodes, such as:

• @graphql-codegen/typed-document-node
• @graphql-codegen/client-preset

Set of changes

• Add { [prop: string]: never } check to variables types