[RFC] @skip/include directive: ability to define default/fallback value

Let's say my app have a React component with an associated GraphQL fragment.

This fragment is used in 10 distinct queries. As of today, field `x` always has to be fetched, but a new business requirement ask to not fetch this field, just for one of the 10 queries.

I can add the following to my fragment:

```graphql
x @skip(if: $skipX)
```

But now, I have to provide a $skipX variable for all of my 10 queries, which seem doable but annoying.

I've like to have a syntax like

```graphql
x @skip(if: $skipX = false)
```

so that I only have to modify the variables of a single query

What do you think about this?

Is there any alternative to my need which does not require modifying all my existing queries?