v2.0 `tbl_summary()` updates

[ddsjoberg]

• [x] Update brdg_*() functions to not rely on the context values (expect for attributes, that is). We want these functions to easily generalize to other functions.
• [x] add_p.tbl_summary()
• [x] add_q() remove verbose messaging from function.
• [x] #1643
• [x] add_overall.tbl_summary()
• [ ] add_ci.tbl_summary()
• [ ] add_stat_label()
• [x] add_stat()
• [ ] separate_p_footnotes() (I think we should add an argument like add_p(footnote_loc = c("header", "body")) to indicate where the p-values go, and we can deprecate this function. But we still need to migrate it, to run its course through the deprecation process.
• [ ] Investigate what I did previously with the percentage in headers stored in .$table_styling$header. Did I scale it? Round it?