Support for wildcard comment

I would like to be able to quickly select all fields on a type. This feature has been [discussed at length](https://github.com/graphql/graphql-spec/issues/127) as an addition to the GraphQL spec, but I think it makes more sense at the code generator level.

## Why is this needed?
My personal use case is simply building a static website on top of NextJS and GraphCMS. However I find myself constantly forgetting to add fields to my queries, and just repeating the work I've done in the CMS. A wildcard syntax would greatly alleviate these problems.

## Proposal
I would suggest two different magic comments, similar to the `import` syntax.

```graphql
aField {
__typename # placeholder field to silence syntax error
# ...all
}

anotherField {
__typename
# ...allScalars
}
```

The first form queries all fields recursively, while the second form limits itself to scalar types.

## Alternatives
[GenQL](https://genql.now.sh) currently supports this via a special `everything` object, however I find JS query builders rather ugly to work with.

## Implementation
I am not a contributor to this project and haven't worked on the backend side of GraphQL, so I'm not sure exactly what the implementation would look like. I wonder if it would be possible to build on top of the introspection plugin.