Series.str.split with expand=True should return DataFrame

**Describe the bug**
```python
s = pd.Series(["abc-def", "ghi-jkl"])
s.str.split("-", expand=True)
reveal_type(s)
```
Above code returns `DataFrame`, but `reveal_type` returns `Series[Unknown]`

**To Reproduce**
1. See above
2. pyright and mypy
3. See above.

Need to modify `pandas-stubs/core/strings.pyi` so that `split` returns `DataFrame` when `expand=True`.  Create proper overload.



