savefiles directory not created on program start

[mwerle]

Follow-up issue to #5917 to track an improvement to the fix. The current implementation delays creating the "savefiles" directory until a game is being saved.

Details in the #5919 PR discussion thread, the TL;DR is below:

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After the fix, there is still a popup "user://savefiles is not a directory" when attempting to load a game if the "savefiles" directory has not yet been created.

Fix by creating the "savefiles" directory on program startup in Pi::App::OnStartup() or as a task inside StartupScreen:Start() in "Pi.cpp".