Support fuzzy matching the Ruby version for `.ruby-version` files

If `asdf-ruby` supported fuzzy matching for `.ruby-version`, it would make it even easier for people to migrate from RVM and chruby. We could remove this disclaimer in the README:

> If you are migrating from version manager that supported fuzzy matching in `.ruby-version` like [rvm](https://github.com/rvm/rvm) or [chruby](https://github.com/postmodern/chruby), note that you might have to change `.ruby-version` to include full version (e.g. change `2.6` to `2.6.1`).

While people _could_ use the [asdf-alias](https://github.com/andrewthauer/asdf-alias) plugin, the experience is not as ideal as having the `asdf` plugin just work.

People can also set `.tool-versions` or override their local copy of `.ruby-version`, but if you don't want to check in changes for a pre-existing project, it's another thing to worry about.

Start with the README section containing the migration disclaimer, then trace how asdf-ruby reads and resolves `.ruby-version` values. Confirm the expected behavior against the RVM and chruby examples in the issue; done means compatible fuzzy versions work without local overrides and the outdated disclaimer can be removed.

Written by the indexing model from the issue text.