traitlets.Set should support generic type annotations for type safety

## Problem

`traitlets.Set` currently lacks generic type support, making it impossible to specify the type of elements it contains for proper type checking.

## Current Situation

```python
from traitlets import Set
import traitlets

class MyClass(traitlets.HasTraits):
# This works but provides no type information
my_set = Set()

# This doesn't work - no generic support
# my_set: Set[str] = Set() # Type error
```

## Desired Behavior

`traitlets.Set` should support generic type annotations similar to how Python's built-in collections work:

```python
from traitlets import Set
import traitlets

class MyClass(traitlets.HasTraits):
# Should be possible for type safety
room_ids: Set[str] = Set()
numbers: Set[int] = Set(default_value={1, 2, 3})
```

## Reference

The `Set` class is implemented in [`traitlets/traitlets.py`](https://github.com/ipython/traitlets/blob/main/traitlets/traitlets.py) as a container trait.

Start in traitlets/traitlets.py at the Set container trait implementation and inspect how its type parameters could correspond to the annotated element type. Done means Set accepts annotations such as Set[str] and Set[int] while preserving the existing default-value behavior; the issue does not name a test file or test entry point.

索引モデルが issue の本文から書いたものです。