webpack-loader: allow loading multiple GraphQL Operations per file

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

When I try to run tests with Jest, using `@graphql-tools/jest-transform`, I got an error with importing multiple GraphQL Operations from a .graphql file. The given error is :

```
GraphQL Webpack Loader allows only for one GraphQL Operation per file

import {deleteMenuMutation, upsertMenuMutation} from 'resources/graphql/menuMutations.graphql'

```

**Describe the solution you'd like**

Support the use of multiple GraphQL Operation per file , as it is intended in graphql-tag/loader
https://github.com/apollographql/graphql-tag/blob/ac4f0da2a3bc07f33312e5af9707a5fb2034edcd/loader.js

**Describe alternatives you've considered**

Separate mutations def in multiple graphql files, but it's not practical for every uses.

**Additional context**

Start at the webpack-loader entry point and trace the error stating that only one GraphQL Operation is allowed per file. Compare its behavior with the linked graphql-tag/loader implementation and the @graphql-tools/jest-transform use case. Done means a .graphql file can expose multiple operations, including deleteMenuMutation and upsertMenuMutation, without that error.

Written by the indexing model from the issue text.