Add support for running tasks on shutdown

[a-palchikov]

This PR adds support for arbitrary tasks to run on shutdown. It is useful when the builder needs to clean up on exit.

I opted to implement it as a special variable with the name Deinit which follows the precedent of Default - let me know if there's a better or more idiomatic way to introduce this inside the template.

[jippi]

Hey! Any chance for this getting some traction again? We're using Magefile at @seatgeek and we could really use this to do proper cleanup of more complex tasks (like docker compose) on exit rather than task specific exit

cc @natefinch

[natefinch]

Yeah, I think it makes sense. Maybe implemented like testing.Tcs Cleanup, so you can treat it like defer, but for the whole process run.

[natefinch]

Yeah, so, I think it would be better to make a mg.Cleanup() method that stores a list of functions to call that are called in a defer in the generated main functions.

The deinit is fine, but it means that deinit doesn't know what code has run. It would be better to have a place where you can shove code as you need it, just like defer.

[jippi]

I like mg.Cleanup a lot better to be honest! :)

[a-palchikov]

Agree on mg.Cleanup - I can take a stab at it in this PR.

[jippi]

Sounds fantastic @a-palchikov - thank you! :)