[troff]: Implement new `.nn` register

...to report the remaining count of lines to have their numbering
suppressed.

* src/roff/troff/env.h (class environment): Declare new member function
  `get_no_number_count()`.

* src/roff/troff/env.cpp (get_no_number_count): Define new member
  function, returning value of member variable `no_number_count`.

  (init_env_requests): Define new ".nn" register and attach it to
  `get_no_number_count()`.

* doc/groff.texi (Miscellaneous): Document it.  Include example of use
  to determine whether current output line will be numbered.  Also
  clarify meaning of register; `.nn` is not decremented except when
  output line numbering is enabled.

* man/groff.7.man (Read-only registers): Document it.

* src/roff/groff/tests/dot-nn_register_works.sh: Test it.
* src/roff/groff/groff.am (groff_TESTS): Run test.

* NEWS: Add item.

See <https://lists.gnu.org/archive/html/groff/2022-11/msg00153.html>.

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