enh: support slicing DataFrame (or Series) with Series

The following should be supported:
```python
import polars as pl

import pandas as pd
import narwhals as nw

df = nw.from_native(pd.DataFrame({'a':[1,2,3], 'b': [4,5,6], 'c': [0,2,1]}), eager_only=True)
print(df[df['c']])  # currently raises
```

Note that for Series this is already supported:
```python
s = df['a']
s[df['c']]  # works fine
```