add Vagrant (missed in 69c03fd)

[MetricMike]

Checklist

• Provide a short description and link to your plugin.

The https://github.com/Banno/asdf-hashicorp plugin supports Vagrant, but it isn't listed in the repository

• Make sure you CI tests for your plugin and they are green.

  If you are using github, you might want to use the
  `plugin_test` action from [asdf-actions](https://github.com/asdf-vm/actions)
  

Covered under asdf-hashicorp's existing tests

• asdf-plugins CI sanity checks are green on your PullRequest.

  You can test locally using:
  
  ```bash
  ./test_plugin.sh --file plugins/PLUGIN_FILE
  ```
  

Covered under asdf-hashicorp's existing tests