Adds data-sort hints to <th> cells so stupidtable.js (see table-sort-init.js) knows which columns are sortable and how to compare their values.
Source:R/utils_render_register_htmls.R
add_sortable_th_attributes.Rdstupidtable.js treats a <th> with no data-sort attribute as unsortable, so this only needs to opt sortable columns in: "string" for plain text/ISO dates (which already sort correctly lexicographically), "int" for the known numeric count columns, and nothing at all for Report/Work, whose cells hold titles/links rather than sortable values, or "Check types" (register#92), whose cells hold a stacked bar with no comparable value.