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

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.