Support type-only TypedDocumentNode declarations

Currently, given a query like this—

```graphql
query findUser($userId: ID!) {
user(id: $userId) {
id
username
role
}
}
```

—the generator creates this output (assuming `typescript-operations` and `near-operation-file` as well for convenience):

```ts
import * as Types from './types';

import { TypedDocumentNode as DocumentNode } from '@graphql-typed-document-node/core';
export type FindUserQueryVariables = Types.Exact<{
userId: Types.Scalars['ID'];
}>;

export type FindUserQuery = { __typename?: 'Query', user?: Types.Maybe<{ __typename?: 'User', id: string, username: string, role: Types.Role }> };

export const FindUserDocument: DocumentNode = {"kind":"Document","definitions":[{"kind":"OperationDefinition","operation":"query","name":{"kind":"Name","value":"findUser"},"variableDefinitions":[{"kind":"VariableDefinition","variable":{"kind":"Variable","name":{"kind":"Name","value":"userId"}},"type":{"kind":"NonNullType","type":{"kind":"NamedType","name":{"kind":"Name","value":"ID"}}},"directives":[]}],"directives":[],"selectionSet":{"kind":"SelectionSet","selections":[{"kind":"Field","name":{"kind":"Name","value":"user"},"arguments":[{"kind":"Argument","name":{"kind":"Name","value":"id"},"value":{"kind":"Variable","name":{"kind":"Name","value":"userId"}}}],"directives":[],"selectionSet":{"kind":"SelectionSet","selections":[{"kind":"Field","name":{"kind":"Name","value":"id"},"arguments":[],"directives":[]},{"kind":"Field","name":{"kind":"Name","value":"username"},"arguments":[],"directives":[]},{"kind":"Field","name":{"kind":"Name","value":"role"},"arguments":[],"directives":[]}]}}]}}]};
```

For many purposes, however, there's zero value in the *actual* TS output, only a type definition, and we in fact want to generate only a `.d.ts` file, which should look something like this:

```ts
import * as Types from './types';

import { TypedDocumentNode as DocumentNode } from '@graphql-typed-document-node/core';
export type FindUserQueryVariables = Types.Exact<{
userId: Types.Scalars['ID'];
}>;

export type FindUserQuery = { __typename?: 'Query', user?: Types.Maybe<{ __typename?: 'User', id: string, username: string, role: Types.Role }> };

export const FindUserDocument: DocumentNode;
```