Route guard properties should not only allow a guard function 

Currently the types for the routes do not restrict the `"?"` property to be a function. This should be the case.
```ts
export interface Routes<T = any> {
  [k: string]: Routes<T> | Resolve<T>;
}
```
I already tried it with a conditional type and with intersection types, but I didn't get it working yet.
PR welcome.