Support NodeJS ES Modules

I'm trying to use named imports from `.mjs` files now that modules are no longer experimental and running into [the same issue that `graphql-tools` had](https://github.com/apollographql/graphql-tools/issues/913):

**gm.mjs**
```js
import { GraphQLModule } from '@graphql-modules/core';
```
```
$ node gm.mjs
(node:27782) ExperimentalWarning: The ESM module loader is experimental.
file:///home/dandv/gm.mjs:1
import { GraphQLModule } from '@graphql-modules/core';
^^^^^^^^^^^^^
SyntaxError: The requested module '@graphql-modules/core' does not provide an export named 'GraphQLModule'
```