Support for `.tool-versions`

I'm not sure if this originated in asdf, but regardless it seems like it could be useful:
https://asdf-vm.com/#/core-configuration?id=tool-versions

Start with the linked asdf core-configuration page to establish the expected .tool-versions behavior, then locate Hermit's configuration-parsing entry point. The issue names no repository files or tests, so the scope and validation criteria need to be clarified before implementation; done should include agreed compatibility behavior and coverage for it.

Written by the indexing model from the issue text.