devel/rubygem-rufo: Add rubygem-rufo 0.18.0

Rufo is as an opinionated ruby formatter, intended to be used via the command
line as a text-editor plugin, to autoformat files on save or on demand.

Unlike the best known Ruby formatter RuboCop, Rufo offers little in the way of
configuration. Like other language formatters such as gofmt, prettier, and
autopep8, we strive to find a "one true format" for Ruby code, and make sure
your code adheres to it, with zero config where possible.

RuboCop does much more than just format code though, so feel free to run them
both!