Unable to use string keys for the proxy.

I'm not able to build the proxy using string keys after it's already a deep object.


```typescript
        data['$' + key] = value
        data[key] = value
```

`Element implicitly has an any type because expression of type string can’t be used to index type DeepSignalObject<{}>. No index signature with a parameter of type string was found on type DeepSignalObject<{}>. (ts 7053)`

