Create a new type category for "internal use" types.

Create a new type category for "internal use" types.

Historically we've put type "char" into the S (String) typcategory,
although calling it a string is a stretch considering it can only
store one byte.  (In our actual usage, it's more like an enum.)
This choice now seems wrong in view of the special heuristics
that parse_func.c and parse_coerce.c have for TYPCATEGORY_STRING:
it's not a great idea for "char" to have those preferential casting
behaviors.

Worse than that, recent patches inventing special-purpose types
like pg_node_tree have assigned typcategory S to those types,
meaning they also get preferential casting treatment that's designed
on the assumption that they can hold arbitrary text.

To fix, invent a new category TYPCATEGORY_INTERNAL for internal-use
types, and assign that to all these types.  I used code 'Z' for
lack of a better idea ('I' was already taken).

This change breaks one query in psql/describe.c, which now needs to
explicitly cast a catalog "char" column to text before concatenating
it with an undecorated literal.  Also, a test case in contrib/citext
now needs an explicit cast to convert citext to "char".  Since the
point of this change is to not have "char" be a surprisingly-available
cast target, these breakages seem OK.

Per report from Ian Campbell.

Discussion: https://postgr.es/m/2216388.1638480141@sss.pgh.pa.us