Syntax Sugar for `._select`

**Is your feature request related to a problem? Please describe.**
`._select` is pretty verbose, especially when forced to use it with keywords like `"from"`

```py
subgraph.Query.snapshots._select("from")
```

**Describe the solution you'd like**
Use the `subscript` helpers for a cleaner syntax (and more intuitive)!

```py
subgraph.Query.snapshots["from"]
```

**Implementation checklist**
- [ ] Add `__setitem__` and `__getitem__`
- [ ] Add tests for those!