[cross_file_platform_interface] Platform interface implementation of cross_file

[bparrishMines]

Platform interface of cross_file.

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Code Review

This pull request introduces the cross_file_platform_interface package, establishing a common platform interface for the cross_file plugin with base classes, creation parameters, and default implementations for file system and scoped storage entities. The review feedback focuses on documentation and API consistency, specifically recommending corrections to class references in doc comments, fixing syntax typos in Dartdoc links, and adding @protected annotations to internal constructors.