typed-document-node: camelCase exports by default?

First of all, thank you for this amazing plugin!

By default it generates TypedDocumentNodes in `PascalCase`. While it makes sense for all generated TypeScript types, it probably should be `camelCase` for nodes as they are values rather than types. For example, `typescript-react-apollo` generates functions that start with lowercase letters.

There's a config option `namingConvention`, but setting it to `change-case-all#camelCase` changes the case of types as well, which is not what we're looking for.

It also looks like `omitOperationSuffix` property doesn't work. Ideally, I would like to see the following being generated:

```ts
export const getUserQuery = ...;
export const deleteUserMutation = ...;
```

Is there a config property I'm missing?