pgsql: Create explain_format.c and move relevant code there.

Create explain_format.c and move relevant code there.

explain.c has grown rather large, so move various functions that
are principally concerned with output generation to a new source
file, explain_format.c, instead of lumping them in with everything
else that is part of explain.c

Reviewed-by: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: http://postgr.es/m/CA+TgmoYutMw1Jgo8BWUmB3TqnOhsEAJiYO=rOQufF4gPLWmkLQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9173e8b604636633a8e3aca54bb56a437bffa718

Modified Files
--------------
contrib/auto_explain/auto_explain.c | 1 +
contrib/file_fdw/file_fdw.c | 1 +
contrib/postgres_fdw/postgres_fdw.c | 1 +
src/backend/commands/Makefile | 1 +
src/backend/commands/explain.c | 704 +--------------------------------
src/backend/commands/explain_format.c | 713 ++++++++++++++++++++++++++++++++++
src/backend/commands/meson.build | 1 +
src/backend/commands/prepare.c | 1 +
src/include/commands/explain.h | 24 --
src/include/commands/explain_format.h | 52 +++
10 files changed, 772 insertions(+), 727 deletions(-)