enh: allow passing Series to Series.__getitem__

[MarcoGorelli]

With Polars we can do:

In [8]: spl = pl.Series([1,2,3])

In [9]: spl[spl[0, 1]]
Out[9]:
shape: (2,)
Series: '' [i64]
[
        2
        3
]

But Narwhals gives an incomprehensible error message:

In [13]: snw = nw.from_native(spl, series_only=True)

In [14]: snw[snw[0, 1]]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[14], line 1
----> 1 snw[snw[0, 1]]

File ~/scratch/.venv/lib/python3.12/site-packages/narwhals/series.py:72, in Series.__getitem__(self, idx)
     70 if isinstance(idx, int):
     71     return self._compliant_series[idx]
---> 72 return self._from_compliant_series(self._compliant_series[idx])

File ~/scratch/.venv/lib/python3.12/site-packages/narwhals/_polars/series.py:126, in PolarsSeries.__getitem__(self, item)
    125 def __getitem__(self: Self, item: int | slice | Sequence[int]) -> Any | Self:
--> 126     return self._from_native_object(self._native_series.__getitem__(item))

File ~/scratch/.venv/lib/python3.12/site-packages/polars/series/series.py:1282, in Series.__getitem__(self, key)
   1254 def __getitem__(
   1255     self, key: SingleIndexSelector | MultiIndexSelector
   1256 ) -> Any | Series:
   1257     """
   1258     Get part of the Series as a new Series or scalar.
   1259
   (...)
   1280     ]
   1281     """
-> 1282     return get_series_item_by_key(self, key)

File ~/scratch/.venv/lib/python3.12/site-packages/polars/_utils/getitem.py:90, in get_series_item_by_key(s, key)
     87     return _select_elements_by_index(s, indices)
     89 msg = f"cannot select elements using key of type {type(key).__name__!r}: {key!r}"
---> 90 raise TypeError(msg)

TypeError: cannot select elements using key of type 'Series': ┌─────────────────────────────────────────┐
| Narwhals Series                         |
| Use `.to_native()` to see native output |
└─────────────────────────────────────────┘

I think we should:

• allow this if the Series is of integral dtypes
• raise if the Series is boolean, advising users to use Series.filter instead

[devdanzin]

I'll give this a try.