add support for other version managers

[SethHorsley]

Would be nice to add support for asdf, or mise or even RVM.

A lot of projects require more than one language being able to have one less file the .ruby-version.

Rbenv doesn't support the .tool-versions or even support the ruby directive from a gemfile which rvm does.

[rafaelfranca]

I was trying to avoid that. The way you deal with multiple languages versions in dev container is different from the .tool-versions. You use the devcontainer features to make sure they are installed with the right versions. I'm pretty sure there is a mise, or asdf feature for devcontainer. If you want to use that, you can use them.

The Ruby feature from Microsoft allows you to install other version managers, and I was trying to not do the same. We can, but I need a better justification than supporting .tools-versions because the right way to manage different languages in devcontainer is to use different features.