Use better default path for Flutter Desktop & Dart

[sync-by-unito]

Currently the default path for Flutter Desktop defaults to current directory. This causes troubles if the application is packaged in an installer and is installed in Program Files directory on Windows for example. In such cases the application does not have write access to the current directory and could only work if it is run as elevated or the developer changes the default path. We believe we should move to the application dir similar to what path_provider enables so packaged Flutter Desktop apps work correctly by default. 

Since path_provider is a Flutter only package we might want to provide a similar implementation without depending directly on path_provider