NLS: Put list of available languages into LINGUAS files

This moves the list of available languages from nls.mk into a separate
file called po/LINGUAS.  Advantages:

- It keeps the parts notionally managed by programmers (nls.mk)
  separate from the parts notionally managed by translators (LINGUAS).

- It's the standard practice recommended by the Gettext manual
  nowadays.

- The Meson build system also supports this layout (and of course
  doesn't know anything about our custom nls.mk), so this would enable
  sharing the list of languages between the two build systems.

(The MSVC build system currently finds all po files by globbing, so it
is not affected by this change.)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/557a9f5c-e871-edc7-2f58-a4140fb65b7b@enterprisedb.com
