Detect missing Rubies and offer to install them

[vinistock]

Note

This issue is aimed at those attending the RubyConf 2024 Hack Day

If we detect a configured Ruby version that isn't installed, we might be able to offer the option for installing it from the VS Code extension. That would be a pretty nice experience.

This won't work for version managers that don't handle Ruby installation as well (like Shadowenv or chruby). But it would work for rbenv, rvm, mise and asdf - as long as detecting that the configured Ruby is missing is straightforward.

[Earlopain]

rbenv doesn't actually come with the build command by default: https://github.com/rbenv/ruby-build
https://github.com/rbenv/rbenv?tab=readme-ov-file#installing-ruby-versions

It would be a good bet but you can't rely on it

[andyw8]

Potentially related: #2583