[ffi] named Arena parameter in using() function

[josxha]

This PR turns the Arena parameter in the using computation function into a named parameter.

While working with the using-pattern, I wished for this tiny quality of life improvement.
I didn't update the changelog since it's such a small change - can add it if wanted.

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[dcharkes]

Nice!

Please also bump the patch version and add a changelog entry so I can hit the release button. 🚀

[github-actions]

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
ffi	None	2.1.4	2.1.5-wip	2.1.4	✔️

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[josxha]

Thanks for your feedback @dcharkes!

• added another parameter name for the releaseCallback(T) in arena.using(). I used res instead of resource for that parameter name because I assume that most implementations of this callback will be one liners.
• bumped the version to 2.1.5
• added changelog entry

[dcharkes]

🙏

• I used res instead of resource for that parameter name because I assume that most implementations of this callback will be one liners.

We try to avoid abbreviations in our code. res is not immediately clear that it's a resource.

[josxha]

My apologies for the slow response on such a small pr.

We try to avoid abbreviations in our code. res is not immediately clear that it's a resource.

I renamed res to resource and checked that the changelog is still up to date.

[dcharkes]

error: pubspec version (2.1.5-wip) and changelog (2.1.5) don't agree

@josxha