Moving nbtree preprocessing code into a new .c file

At one point during my work on nbtree SAOP scans (during the Postgres
17 cycle), Heikki suggested that I move all nbtree preprocessing code
into its own file. For whatever reason I didn't follow-up on his
suggestion at the time. The nbtree skip scan patch will add quite a
bit more preprocessing code, though, so now seems like a good time to
follow up -- better late than never.

Attached mechanical patch relocates all code that only runs during
nbtree preprocessing into a new file, nbtpreprocesskeys.c. I verified
that the changes in the patch were strictly mechanical using "git diff
--no-ext-diff --color --color-moved=dimmed-zebra master".

Note that there was some minor reordering of the functions in the new
nbtpreprocesskeys.c, relative to their original nbtutils.c order. The
reordering puts _bt_preprocess_array_keys and related routines towards
the end of the file (not at the start, as in nbtutils.c). Naturally,
nbtpreprocesskeys.c starts with _bt_preprocess_keys() itself, since
that is the only externally callable function. There was also some
minor reordering within _bt_preprocess_keys-owned functions, to make
the order consistent with the actual order in which each function is
first called by _bt_preprocess_keys (I wouldn't usually bother with
this kind of reordering, but it's worth doing now).

--
Peter Geoghegan