Feat: Compile native gem(s)

[retorted]

Adds a GH workflow to automatically compile and push native gem versions for x86_64-linux and arm64-darwin when a tag is pushed.

Setup

• Merge the PR (I think workflows only become active after they're merged to main)
• Add RUBYGEMS_API_KEY to github secrets for the repo

Usage

• Prep a release version, eg: 0.2.1
• Build and publish the regular version of the gem in whatever way you do it now
• Create and push a new tag, eg: git tag -a 0.2.1 && git push origin 0.2.1
• The workflow should run jobs to create native versions of the gem(s) and push them to rubygems automatically

Notes

• I initially tried using the docker-based setup provided by oxide-rb but ran into various issues here for reference.
• There are some annoying formatting issues around building for Darwin at the moment; when invoking the task you have to specify the Darwin version in a suffix without a hyphen, eg rake native:arm64-darwin23, the output of that task creates a file with a hyphenated suffix (*-arm64-darwin-23.gem) but apparently the suffix is basically pointless and should be stripped out before publishing, eg *-arm64-darwin.gem. Related discussion here: Make no distinction between arm64-darwin-{21,22,23} ruby/rubygems#7407