[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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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/objective_c/lib/src/ns_input_stream.dart

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources
ffi	MallocAllocator	allocation.dart::malloc
ffi	CallocAllocator	allocation.dart::calloc

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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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Changelog Entry ❗

Package	Changed Files
package:ffi	pkgs/ffi/lib/src/arena.dart

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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