[cross_file] exists() and delete() file implementation

[hossameldinmi]

Implementing exists() and delete() functions for Native and HTML

A simple solution for this feature request, 177475

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request implements exists() and delete() methods for XFile on both native and web platforms. The changes include adding the method definitions to the base class, providing implementations for dart:io and dart:html, and adding corresponding tests. My review focuses on improving the correctness and readability of the web implementation.