internal: add native_sub.c for sys package

[Young-Flash]

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[peter-jerry-ye-code-review]

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After reviewing the git diff, here are three potential issues or observations:

1. File Extension Inconsistency:

   • The file fs_native.mbt uses .mbt extension while ffi.mbti uses .mbti extension.
   • This might cause confusion about file type conventions in the project. It would be good to maintain consistency in file extensions unless there's a specific reason for the difference.

2. Path Handling in Native Code:

   • In native_stub.c, the code uses platform-specific path separators (windows.h vs dirent.h).
   • There's no explicit handling of path separators conversion between different platforms, which could potentially cause issues when dealing with file paths across different operating systems.

3. Error Handling:

   • The environment variable functions (set_env_var, unset_env_var) don't appear to have any error handling or return values to indicate success/failure.
   • This could be problematic if these operations fail, as the calling code won't know about the failure.

These are potential areas that might need attention depending on the project's requirements and use cases.