[cross_file] Add runnable example with main()

[momshaddinury]

Adds a main() entry point and an XFile.fromData demonstration to the cross_file example, so pub.dev's Pana scoring recognises that the package ships with an example.

The existing readme_excerpts.dart defined instantiateXFile() but had no top-level main(), so the file wasn't discovered as an example. This keeps the existing #docregion Instantiate block used by README.md and adds a second #docregion InstantiateFromData block alongside a main() that runs both excerpts.

The new test exercises XFile.fromData on every platform (the existing path-based test is skipped on web, which has no filesystem).

Fixesflutter/flutter#93073

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[gemini-code-assist]

Code Review

This pull request updates the cross_file package example by adding a runnable main entry point and a demonstration of XFile.fromData for in-memory byte construction. Corresponding tests were added to verify the new functionality. Feedback includes removing redundant type arguments in the byte list initialization and wrapping the disk-based file instantiation in a try-catch block to improve robustness across platforms like the web.

[stuartmorgan-g]

Thanks for the contribution!

We want examples to be easily discoverable by users, and also to show up on pub.dev, so a main entry point for an example should be in lib/main.dart. If we only need one file, the file should be renamed.

[momshaddinury]

@stuartmorgan-g I was caught up with another PR. Will address the feedback this week.

[momshaddinury]

@stuartmorgan-g, is there any update to this issue? LMK if there's any feedback.

[bparrishMines]

LGTM

[momshaddinury]

@bparrishMines, can you please add the CICD label again?

[momshaddinury]

@bparrishMines fixed the failed readme snippet validation. Resolved the conflict. Please add the CICD label again. Thanks.

[bparrishMines]

@stuartmorgan-g For the final review