Make pack() and unpack() generic

[krlmlr]

Useful now that duckdb supports them with the DBI interface: duckdb/duckdb#3495.

[krlmlr]

I wonder if we should instead define new generics build_pack_spec() and pack_spec(), similarly for "unpack". The DuckDB implementation is copying a bit of code from tidyr: duckdb/duckdb#3829.

[olivroy]

I may be mistaken, but I think this would help separate_wider_delim() / separate_wider_regex() work with sf objects. Is there a plan to merge this PR? (r-spatial/sf#2359)

[elipousson]

Adding to @olivroy's comment, it would be great to see this PR reviewed and merged to support the implementation of separate_wider_delim() / separate_wider_regex() with sf.

I'm teaching sf to a graduate course this fall and seeing these functions silent dropping the sf class (when 90+% of tidyverse functions work seamlessly) is such a weird and potentially frustrating loophole!