chore: check in .tool-version

On my machine Vagrant needs the ruby gems installed via my package manager.

However, my editor needs `ruby-lsp` for `.rb` files. I have to install this
binary. I prefer to use [asdf-vm](https://github.com/asdf-vm/asdf) and keep my
shell config files clean of `PATH` overrides.

I think, specifying the ruby version to `system` is a good solution for Vagrant
here.

deployment/local-testing/.tool-versions

@@ -0,0 +1 @@
+ruby system