Support hex (Elixir / Erlang)

[mattwynne]

🤔 What's the problem you're trying to solve?

Making releases of cucumber-messages for elixir

✨ What's your proposed solution?

Borrow code from here to implement something that works on the project's mix.exs file

📚 Any additional context?

cucumber/action-publish-hex#1 will contain the code to actually publish to hex; all we need to do here is read / update version numbers in the mix.exs file.

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