Add .$table_styling$header$modify_stat_* for tbl_strata() levels ?

[ddsjoberg]

When we run tbl_strata() it would be helpful to store more information about the stratum levels.

For example, if the stratum were trt and grade, then we could possible keep the following data:
modify_stat_strata_levels: "Drug A, II"
modify_stat_strata1_level: "Drug A"
modify_stat_strata2_level: "II"
modify_stat_strata1_var: "trt"
modify_stat_strata2_var: "grade"

Perhaps also the Ns, but would need to think clearly about what those Ns would be and how they are nested/not nested
modify_stat_strata1_n, modify_stat_strata2_n

This could be helpful for functions like tbl_heirarchical() where we want to perhaps use tbl_strata() in the background to build tables split by both grade and treatment.