[in-app-purchase] Adopt code excerpts in README

[mike-v2]

Improves README example and updates it to use code excerpts. Fix wording in README Add comments to example/main to aid README explanation Create readme_excerpts file for platform-specific code that is not present in example/main

Part of flutter/flutter#102679

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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[mike-v2]

I'm not sure if making a test for readme_excerpts.dart would be useful since it's just accessing properties from data that's retrieved from an API call.

[stuartmorgan-g]

@mike-v2 I see there are new commits here since my review; is this ready for re-review?

[stuartmorgan-g]

Thank you for your contribution. I'm going to close this PR for now since there are outstanding comments, just to get this off our PR review queue. Please don't hesitate to submit a new PR if you have the time to address the review comments. Thanks!