Administration: Improve performance of List Tables.
Improve the performance of WP_List_Table::get_column_info() by adding the primary column to the cached header values. This reduces the number of calls to the WP_List_Table::get_primary_column_name() method to once per table in line with the other header getter functions.
Props bobbingwide, chaion07, costdev, mikeschroder, mukesh27, peterwilsoncc, shetheliving, spacedmonkey.
Fixes #34564.