Refactor: factor out PolarsDataFrame

[MarcoGorelli]

The current structure is:

.
└── narwhals
    ├── _arrow
    ├── _pandas_like
    └── stable

and differences between different Polars versions are handled directly in narwhals.dataframe.py and narwhals.series.py. I think it'd be cleaner if there was an extra _polars directory, i.e.:

.
└── narwhals
    ├── _arrow
    ├── _pandas_like
    ├── _polars
    └── stable

and that differences between Polars versions could all be handled in there

We could probably then remove _is_polars and _backend_version from narwhals.DataFrame

[FBruzzesi]

Closed by #561