Generate only queries or only mutations

**Is your feature request related to a problem? Please describe.**

I am trying to use a different fetcher for queries and mutations. Perhaps this is a more that I am trying to solve my problem in the wrong way, although still it would be nice to be able to generate _only_ queries or _only_ mutations.

**Describe the solution you'd like**

I would imagine it would be a configurable option in the config that takes "queries" or "mutations".

```yaml
documents: src/**/*.graphql
generates:
src/generated/queries.ts:
plugins:
- typescript
- typescript-operations
- typescript-react-query
config:
fetcher: "../codegen.fetcher#queryFetcher"
generate: "queries"
src/generated/mutations.ts:
plugins:
- typescript
- typescript-operations
- typescript-react-query
config:
fetcher: "../codegen.fetcher#mutationFetcher"
generate: "mutations"
```