TYP overload fillna
`fillna` has a different return type according to whether `inplace` is `True` or `False`. We could overload it to provide a better typing experience

See https://github.com/pandas-dev/pandas/pull/40197 for a very similar issue

Task here is:

1. read through #40197, make sure you understand it
2. try doing the same for `fillna`
3. check it works. See the above PR for an example, but it should be something like the following (but for `fillna` instead of `reset_index` - some of the other arguments probably need changing too)

```python
# t.py

import pandas as pd

inplace: bool

reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0))
reveal_type(pd.DataFrame([1,2,3]).reset_index(inplace=False))
reveal_type(pd.DataFrame([1,2,3]).reset_index(drop=True, inplace=True))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, inplace=True))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, drop=False, inplace=True))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, inplace=True, col_fill='a'))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, col_fill='a'))
reveal_type(pd.DataFrame([1,2,3]).reset_index(inplace=True))
reveal_type(pd.DataFrame([1,2,3]).reset_index(inplace=inplace))
reveal_type(pd.DataFrame([1,2,3]).reset_index(drop=True, inplace=inplace))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, inplace=inplace))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, drop=False, inplace=inplace))
reveal_type(pd.DataFrame([1,2,3]).reset_index(level=0, inplace=inplace, col_fill='a'))
reveal_type(pd.DataFrame([1,2,3]).reset_index(inplace=inplace))
```

output:
```python
$ mypy t.py 
t.py:5: note: Revealed type is 'pandas.core.frame.DataFrame'
t.py:6: note: Revealed type is 'pandas.core.frame.DataFrame'
t.py:7: note: Revealed type is 'None'
t.py:8: note: Revealed type is 'None'
t.py:9: note: Revealed type is 'None'
t.py:10: note: Revealed type is 'None'
t.py:11: note: Revealed type is 'pandas.core.frame.DataFrame'
t.py:12: note: Revealed type is 'None'
t.py:13: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
t.py:14: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
t.py:15: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
t.py:16: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
t.py:17: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
t.py:18: note: Revealed type is 'Union[pandas.core.frame.DataFrame, None]'
```

---

I've marked this as 'good first issue' as it doesn't require any knowledge of the pandas core codebase, though it's probably not the easiest if you're not at least somewhat familiar with mypy / typing