Support SQL/PSM-compatible CASE statement in plpgsql.

doc/src/sgml/errcodes.sgml

doc/src/sgml/plpgsql.sgml

src/include/utils/errcodes.h

src/pl/plpgsql/src/gram.y

src/pl/plpgsql/src/pl_exec.c

src/pl/plpgsql/src/pl_funcs.c

src/pl/plpgsql/src/plerrcodes.h

src/pl/plpgsql/src/plpgsql.h

src/pl/plpgsql/src/scan.l

src/test/regress/expected/plpgsql.out

src/test/regress/sql/plpgsql.sql