Add file.Copy function to copy files with permissions

[piraz]

Implement a new function Copy in the file package to copy a file from a source path to a destination path, preserving file content and permissions.

Acceptance Criteria

• Add Copy(src, dst string) error to the file package.
• The function should:
  • Copy the file's content from src to dst.
  • Overwrite dst if it exists, or create it if it doesn't.
  • Preserve the file mode (permissions) from the source file.
  • Return an error if the operation fails.
• Add GoDoc documentation for the function.
• Include basic unit tests (using testify) to verify correct behavior.

Example usage:

err := file.Copy("foo.txt", "bar.txt")
if err != nil {
    log.Fatal(err)
}

This utility will make it easier to copy files in Go projects using fsok.