This gem was bundled in main/ruby and it's on the list of the "Bundled gems"
(https://stdgems.org/#bundled-gems-ruby-302) for Ruby 3.0.2. I'm moving it
to a separate aport to provide a correct pkgver - to avoid confusion and
future complications if the upstream decide to remove it from the
"Bundled gems" (as they did for some other gems with 3.0.0 release).

When the gem was bundled its pkgver was set to that of the ruby package,
now its pkgver is set to its upstream version which is lower so users
upgrading need to pass `--available` to the `apk upgrade` command.