feat: add exercism plugin

[bheesham]

Summary

Description:

• Tool repo URL: https://github.com/exercism/cli
• Plugin repo URL: https://gitlab.com/bheesham/asdf-exercism

Checklist

• [X] Format with scripts/format.bash
• [X] Test locally with scripts/test_plugin.bash --file plugins/<your_new_plugin_name>
• [X] Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI