Add a common `FileHelper` class

[AndreasReitberger]

Add a helper class to save and open files from a Maui project.

• 1: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/maui/essentials/file-saver?tabs=windows
• 2: https://learn.microsoft.com/en-us/dotnet/maui/platform-integration/appmodel/launcher?view=net-maui-7.0&tabs=macios#open-another-app-via-a-file