Add value stacking capability

[tylerriccio33]

nw.concat_str(nw.col("n_unique"), nw.lit("<br>"), nw.col("__frac_n_unique")

In the code above I take 2 columns, n_unique and __frac_n_unique and insert a break between them. It would be nice to do this w/great_tables itself and not that code. Maybe that code can just be inserted into the great tables code base :)?

Maybe something like GT().stack_values(...)?

I think the R package has something similar, or at least there was some recipe for this back when I was using R.