Remove redundant validation when enumerating files & folders in SystemFile/SystemFolder.

[itsWindows11]

Currently, when enumerating or copying files, we are checking whether the file exists or not in the constructor. This can cause unnecessary huge disk I/O overhead especially with large folders.

A new internal constructor is created specifically to prevent this scenario (only creates an instance with the necessary item info, nothing else), since in these cases we already know that the file/folder indeed exists.

cc. @HEIC-to-JPEG-Dev feel free to review.

[Arlodotexe]

This is a great change. The constructor validation/guard is largely meant for consumers who might provide an invalid or nonexistent path. A file or folder resource should exist before it's accessed through our abstraction.

That means instances that are created internally can pre-validate this and skip the consumer-facing validation/guard.

It's a free performance gain that might be especially noticeable for extremely large storage graphs.

My only suggestion is that we rename this from isInternal to something more descriptive of what it does. Maybe we just call it noValidation and keep the constructor internal?

[itsWindows11]

I've merged the PR with the requested changes.

[Arlodotexe]

Great work here, we'll get this out in the next version bump.

[itsWindows11]

Just added even more optimizations that should get the enumeration time in tests to less than 1ms in most cases. Using StorableType.All mode we are now using the DirectoryInfo to enumerate file system info entries, which makes it easier to distinguish between files and folders by just accessing the attributes instead of another File.Exists() call.

[Arlodotexe]

New changes look good!