Make vm.args explicit

[DianaOlympos]

Summary of changes

To make the vm.args file more explicit to the user, we create a default vm.args that we put at the root of the rel folder through release.init. Additionaly we make rel/config.exs use it.

We also provide an update for OTP21+ for the args.

We keep the old vm.args for backward compatibility for project already using distillery.

There was a couple of formatting things added, i can get them out and open another PR if needed for them.

Checklist

• [ x] Same for documentation, including moduledocs
• [x ] Tests were added or updated to cover changes
• [x ] Commits were squashed into a single coherent commit

Licensing/Copyright

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[bitwalker]

Thanks! I like that this is now explicit, rather than generated from thin air :)

[DianaOlympos]

Yeah that was the goal. Should make it easier for users to know where it come from and understand what they can do now :)