devtools_shared: Use the file package for a file system abstraction for testing

[srawlins]

Mainly, the extension LocalFileSystem on Never is ripped out for an extension on the 'file' package's FileSystem. Production code can use a top-level fileSystem constant, which is the local file system. Test code can use a MemoryFileSystem. This avoids reading and writing files on devs' machines and CI machines. Better isolation. From the CHANGELOG:

• Braking changes: LocalFileSystem, an extension which provided some handy
  helpers, has been refactored into an extension on the file package's
  FileSystem abstraction. In detail:
  • fileSystem is a new top-level constant which represents the local (real)
    file system.
  • LocalFileSystem.devToolsDir() is now a static getter,
    FileSystemExtension.devToolsDir.
  • LocalFileSystem.maybeMoveLegacyDevToolsStore() is now an instance method,
    FileSystemExtension.maybeMoveLegacyDevToolsStore().
  • LocalFileSystem.devToolsStoreLocation() is now a static getter,
    FileSystemExtension.devToolsStoreLocation.
  • LocalFileSystem.ensureDevToolsDirectory() is now private.
  • LocalFileSystem.devToolsFileFromPath() is now an instance method,
    FileSystemExtension.devToolsFileFromPath().
  • LocalFileSystem.devToolsFileAsJson() is now an instance method,
    FileSystemExtension.devToolsFileAsJson().
  • LocalFileSystem.flutterStoreExists() is now an instance getter,
    FileSystemExtension.flutterStoreExists.
  • IOPersistentProperties.new accepts a new optional FileSystem fs
    argument.
• Update LocalFileSystem and IOPersistentProperties to use package:file
  instead of dart:io to allow mocking the file system.

[gemini-code-assist]

Code Review

This pull request refactors LocalFileSystem and IOPersistentProperties in packages/devtools_shared to use package:file instead of dart:io, enabling file system mocking in tests. Feedback on the changes includes a recommendation to allow passing a custom FileSystem to DevToolsUsage to prevent real disk writes during tests, a suggestion to defensively check the decoded JSON type in devToolsFileAsJson to avoid runtime type errors, and a minor correction for a typo in the changelog.