perf: make sure X.schema is only calculated once per dataframe in TypeSelector

[MarcoGorelli]

Description

Refactors TypeSelector to use Narwhals selectors, as opposed to hard-coding types

This is preferable because, for Polars LazyFrames, getting the schema isn't a free operation (especially if the original dataframe is on the cloud - LazyFrame.schema may get renamed to LazyFrame.collect_schema for this reason)

I've also expanded the tests

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [ ] My code follows the style guidelines (ruff)
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation (also to the readme.md)
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added tests to check whether the new feature adheres to the sklearn convention
• [ ] New and existing unit tests pass locally with my changes

[MarcoGorelli]

I have one concern thought: I realized that polars selectors select in the order they are given and do not maintain the original order.

that's a good point, thanks - it should be possible to preserve the order. df.schema needs calling here anyway - i'll update, making sure that df.schema is called no more than once per dataframe