[wasm][bcl] Handle correctly backslash in file name (#34746)

[wasm][bcl] Handle correctly backslash in file name (#34746)

- Modify IO file to assign backslash to alternate directory separator by default
- Add tests for issue https://github.com/mono/mono/issues/18933

closes: https://github.com/mono/mono/issues/18933

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Co-authored-by: kjpou1 <kjpou1@users.noreply.github.com>