[ffigen] Migrate ObjC example to Dart API

[liamappelbe]

I'm doing this as part of updating https://dart.dev/interop/objective-c-interop. Migrating the rest of the yaml configs is lower priority.

Also, add xcodePath, iosSdkPath, and macSdkPath to the public API. These getters expose the variable substitutions available in the YAML config: $XCODE, $IOS_SDK, and $MACOS_SDK.

[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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[coveralls]

coverage: 89.778% (+0.02%) from 89.76%
when pulling 752aebb on ffigen_example
into b17023d on main.