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!

Look at the `_select` method in the codebase, likely in a class like `Query` or `Field`. Implement `__getitem__` and `__setitem__` to allow subscript syntax. Write tests to ensure the new syntax works correctly, especially with reserved keywords like 'from'. Check existing tests for `_select` to understand the expected behavior.

索引モデルが issue の本文から書いたものです。