[shared_preferences] Add shared_preferences_windows

[stuartmorgan-g]

Description

Dart implementation of the shared_preferences plugin using ffi for Windows.

This is #2631, trimmed down to the initial stand-alone PR that adds shared_preferences_windows, since it must be landed and published before the shared_preferences registration and endorsement can land.

Related Issues

flutter/flutter#41719

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[stuartmorgan-g]

This is now working, and passing the experimental CI bots. However, looking again at the way it's doing path storage, and comparing with the Linux implementation, it makes a lot more sense to hold off on this until path_provider_windows is landed, and then use it to get the storage location instead of having the Win32 code duplicated here (with different logic for constructing the subpath, as compared to path_provider). Once path_provider_windows is in I'll rework this to depend on it, as we do with shared_preferences_linux.

[stuartmorgan-g]

@franciscojma86 Can you approve the co-authorship here when you get a chance? I've done a fair amount of rewriting based on what we learned from some of the Linux plugins, especially aligning it with the basic structure of shared_preferences_linux due the fact that path_provider_windows is now mostly complete, but it's still very much based on your PR.

[franciscojma86]

@googlebot I consent.

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[franciscojma86]

So exciting!

[stuartmorgan-g]

This is essentially done, it just needs a trivial pubspec updated after path_provider_windows 0.0.2 is published (#2818).

[csells]

I don't think we're calculating the path where the shared preferences are stored correctly.