add fs

[lijunchen]

No description provided.

[peter-jerry-ye-code-review]

1. Potential Typos and Inconsistent Function Naming:

   • In fs/fs.mbti, the function write_string is defined, but in fs/fs.wasm-gc.mbt, an internal function _write_string is used. These should ideally be consistent to avoid confusion.
   • Similarly, read_to_string in fs/fs.mbti maps to _read_to_string in fs/fs.wasm-gc.mbt. Consider using the same function names unless there's a specific reason for the distinction.

2. Incomplete Documentation:

   • The documentation in fs/fs.wasm-gc.mbt for _write_string is missing. It should include a complete description, parameters, and return values similar to other documented functions in the same file.

3. Error Handling in Function Calls:

   • In fs/fs.wasm-gc.mbt, the function read_to_string checks if the file exists using exists(~path) and then calls _read_to_string(path). However, it might be more robust to include explicit error handling in _read_to_string to manage any potential issues encountered during file reading operations, not just the existence check.