Test use of polars selectors throughout library

[machow]

Related to https://github.com/posit-dev/great-tables/pull/34, I've implemented the path for using polars selectors, but need to add more tests of its usage. It seems like aspects of selection can be consolidated in places (e.g. the handling of columns in the cols_* functions).

Most functions that take a selection argument do a bit of custom checking, and these checks seem largely the same across functions..!

[machow]

To check:

• select via scalar (e.g. "some_col")
• select via list
• select via polars
• select via lambda
• (what should happen on a list of polars selectors?)