psql: Make \d+ partition list formatting consistent with other objects

psql: Make \d+ partition list formatting consistent with other objects

Previously, \d+ <table> displayed partitions differently from other object
lists: the first partition appeared on the same line as the "Partitions"
header. For example:

    Partitions: pt12 FOR VALUES IN (1, 2),
                pt34 FOR VALUES IN (3, 4)

This commit updates the output so that partitions are listed consistently
with other objects, with each entry on its own line starting below the header:

    Partitions:
        pt12 FOR VALUES IN (1, 2)
        pt34 FOR VALUES IN (3, 4)

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Neil Chen <carpenter.nail.cz@gmail.com>
Reviewed-by: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Soumya S Murali <soumyamurali.work@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHut+Pu1puO00C-OhgLnAcECzww8MB3Q8DCsvx0cZWHRfs4gBQ@mail.gmail.com