dist: add dist-zstd option

dist: add dist-zstd option

Add support for using the zstd compression algorithm.
Use a default compression setting of -19, and ".zst" as the suffix.

* bin/automake.in (handle_dist): Add zstd to the list of known dist-
suffixes.
(preprocess_file): Map ZSTD to dist-zstd.
* doc/automake.texi: Document the new option.
* lib/Automake/Options.pm (_is_valid_easy_option): Add dist-zstd.
* lib/am/distdir.am (dist-zstd): New rule.
(?ZSTD?DIST_TARGETS): Add definition.
(distcheck): Add a case for *.tar.zst*.
* t/dist-formats.tap: Add tests.
* NEWS: Mention the change.