GAP-33: A proposal for Set-semantic compatible Type System Definitions (#33)

I have been working on a lot of operations that benefit from treating
the GraphQL Schema as a Set. For context, see changes in the Relay
schema-set crate:
https://github.com/facebook/relay/tree/main/compiler/crates/schema-set

One of the core issues is there's no syntax I can use to get partial
sets. For example I can have a type-definition and field definition
removed, but there is still an argument on the field present.

This proposes to solve that through syntax that looks like:
```
extend type Person {
  extend field(argument: String) @deprecated
}
```

We could get into this state if we did something like:
```
type Person {
  field(argument: String): String @deprecated
}
```
**exclude**
```
type Person {
  field: String
}
```

This proposal should give a *syntax* that we could use in @egoodwinx's
proposal in #4

Author: magicmark