support ruby-head by using stable-api-compiled-fallback feature of rb-sys

[davekaro]

I want to use mrml-ruby in my project, but was concerned it would block me from updating to Ruby 3.4 based on https://github.com/hardpixel/mrml-ruby/issues/12. Looking around from the comment on that issue, I found out about the stable-api-compiled-fallback feature of rb-sys (see https://github.com/oxidize-rb/rb-sys/pull/229 for details).

I am not familiar with rust tooling or how to use rust packages within ruby, but piecing together from similar PRs in similar projects, I wanted to submit this PR as a starting point. I referenced these PRs:

https://github.com/gjtorikian/commonmarker/pull/288 https://github.com/IAPark/tiktoken_ruby/pull/14