feat(api): `Config.resolve()` can return `Promise`

BREAKING CHANGE: The `Config.resolve()` method can return a `Promise` if any
underlying loader or resolver has returned a `Promise`.

It is recommended to assume it returns a `Promise` and always `await` the
result:

```diff
-const resolved = config.resolve();
+const resolved = await config.resolve();
```

If you need synchronous code ensure the configuration, the loader and the
resolver all returns synchronous results, e.g. the `staticResolver` with
synchronous code.