[swift2objc] Add support for mutating functions

[nikeokoronkwo]

Fixes #1737

The changes here address support for mutating functions.
Not much needs to be done in the transformation end, as mutating functions are only allowed on structs and protocols.

[github-actions]

PR Health

Breaking changes ✔️

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Changelog Entry ✔️

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Changes to files need to be accounted for in their respective changelogs.

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.

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License Headers ✔️

// 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
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[liamappelbe]

Needs formatting.

[coveralls]

coverage: 84.595% (-2.6%) from 87.151%
when pulling 5c2edc1 on nikeokoronkwo:swift2objc-mutating
into 440c0e7 on dart-lang:main.