[ffigen] add example from readme as real example for reference

[goderbauer]

This example was introduced in the "Getting Started" section of the readme, see #2691. This PR adds that example to the example subdirectory so we can link to it from the readme. People who get stuck can then simply checkout the example to figure out what they did wrong.

TODO: Update readme after #2691 lands to link to this.

[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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[goderbauer]

Failure seems to be coming from a missing dart pub get in the new example.

@liamappelbe where do we run that for the other examples?

[dcharkes]

Do we require this SDK version?

[goderbauer]

Nah, this was just generated by dart create. I updated it to match what the other examples do.

[dcharkes]

We should be running analyze etc against stable as well, so that should have made the CI red.

[goderbauer]

That failure was probably hidden by the failure caused by not running dart pub get at all in this new example. See my comment above.

[coveralls]

coverage: 86.719% (-3.0%) from 89.749%
when pulling 815b2c8 on goderbauer:ffigenexample
into c2cad10 on dart-lang:main.