lang/ruby31: Add new port

Add preview1 of Ruby 3.3 series.

To handle pre-release version, Mk/bsd.ruby.mk is changed as following.

* New variable RUBY_DISTVERSION is introduced. It keeps DISTVERSION of
  each ruby port (lang/rubyXY).
* The value of RUBY_VERSION, RUBY_PORTVERSION, RUBY_WRKSRC and
  RUBY_DISTNAME is set by using that of RUBY_DISTVERSION

ReleaseNotes:	https://www.ruby-lang.org/en/news/2021/11/09/ruby-3-1-0-preview1-released/
PR:		259746
Approved by:	maintainer timeout (> 1 month)