Add function to display all ID values/labels for a gt table

[rich-iannone]

It can become increasingly difficult to recall the ID values associated with different labels in a gt table. There are also processes where gt will generate ID values (e.g., tab_spanner_delim(), etc.) while ensuring that duplicate ID values aren't produced. For the latter case, it is impossible to know what those ID values are unless one were to examine to correct component of the gt_tbl object.

Because it's important to know these ID values for targeting (for styles, creating borders, adding footnote marks), we should have a convenient function (tab_info()) that summarizes (by location) the ID values and their labels.