NLS: Put list of available languages into LINGUAS files

This possible change was alluded to in the meson thread at [0].

The proposal is to move 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.)

In practice, the list of languages is updated mostly by means of the
cp-po script that I use to update the translations before releases [1],
and I have a small patch ready for that to adapt to this change. (In
any case, that wouldn't be needed until the beta of PG16.)

[0]:
https://www.postgresql.org/message-id/bfcd5353-0fb3-a05c-6f62-164d98c5689d@enterprisedb.com

[1]:
https://git.postgresql.org/gitweb/?p=pgtranslation/admin.git;a=blob;f=cp-po;h=d4ae9285697ba110228b6e01c8339b1d0f8c3458;hb=HEAD