Allow function fields to be generic

```ts
/** @gqlField */
export function nodes(connection: Connection): T[] {
return connection.edge.map(edge => edge.node);
}
```

Should act as a functional equivalent of a method on a generic class:

```ts
/** @gqlType */
class Connection {
// ... other fields/methods
nodes(): T[] {
return this.edges.map(edge => edge.node)
}
}
```