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@bowt.ie>
Discussion: http://postgr.es/m/CA+TgmoYutMw1Jgo8BWUmB3TqnOhsEAJiYO=rOQufF4gPLWmkLQ@mail.gmail.com

src/backend/commands/meson.build

@@ -22,6 +22,7 @@ backend_sources += files(
   'dropcmds.c',
   'event_trigger.c',
   'explain.c',
+  'explain_format.c',
   'extension.c',
   'foreigncmds.c',
   'functioncmds.c',