Feature Request: Add support for async scalar resolvers

Could you add support for async scalar resolvers, e.g. add optional `encodeAsync` and `decodeAsync` methods:

```
export type ScalarResolver = {
  encode?: (s: unknown) => string;
  encodeAsync?: (s: unknown) => Promise<string>;
  decode?: (s: unknown) => unknown;
  decodeAsync?: (s: unknown) => Promise<unknown>;
};
```