[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]

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

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

[liamappelbe]

Needs formatting.

[liamappelbe]

It'd be cleaner to do this in parseFunctionInfo, and add a flag to ParsedFunctionInfo, even though not all kinds of function support mutating.

That will also mean you can test it in test/unit/parse_function_info_test.dart.