More tbl_stack() grouping options

[ddsjoberg]

at the moment, when we stack tables with headers, there is just one header type that utilizes the the gt groupname_col option. When the table is not exported via gt, the grouping column is placed as a new column.

I want to update to include an option to add headers and nest the tables underneath and a second option to put the header in a new column (like the non-gt export of the default).

https://insightsengineering.github.io/tlg-catalog/stable/tables/efficacy/coxt02.html

This would make it easier to group the primary treatment variables from the covariates.