[browser][io] Workaround for issue MoveDirectory (#40310)

[browser][io] Workaround for issue MoveDirectory (#40310)

* [browser][io] Workaround for issue MoveDirectory

- Issue workaround while waiting for emscripten fix.  https://github.com/dotnet/runtime/issues/40305
- The following code checks for the existence of the source and destination directories which replaces the same code in the emscripten code.

emscripten tracking issue:  https://github.com/emscripten-core/emscripten/issues/11804

* Update src/libraries/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs

Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>
Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>