Support set operations on selectors

[MarcoGorelli]

We currently have selectors: https://narwhals-dev.github.io/narwhals/api-reference/selectors/

E.g. you can do

df.select(nw.selectors.numeric())

but you can't currently do

df.select(nw.selectors.numeric() - nw.selectors.by_dtype(nw.Int64))

This is fairly important, as:

• df.select(nw.selectors.numeric(), ~nw.selectors.by_dtype(nw.Int64)) might raise due to duplicate column names
• using df.schema isn't free on a lazyframe