api: plan for narwhals.stable.v2

[MarcoGorelli]

I think we can aim for narwhals.stable.v2 some time in 2025

The main things I'd like to achieve are:

• remove "interchange" level of support, and instead only have:
  • "full": must support Series and have a defined row order (pandas, polars.DataFrame, pyarrow.Table, modin.DataFrame, cudf.DataFrame)
  • "lazy": must support collect, adding/removing/subsetting columns, filtering rows. Initially, at least, row-order-dependent operations (such as nw.cum_sum) will be left out. This will include Polars.LazyFrame, DuckDBPyRelation, pyspark.DataFrame, ibis.Table, dask.DataFrame
• make the breaking changes listed in breaking changes

Candidate changes:

• rework from_native. It will get simpler once we remove interchange-level things, but it's still fairly complicated. Not sure how much I like it to be completely honest

Usage of narwhals.stable.v1 should remain unaffected