Add Platform-Specific Option to Enable Snackbar on Windows

[brminnick]

Snackbar now requires additional code to be added on Windows to the Package.appxmanifest file: MicrosoftDocs/CommunityToolkit#374

Currently, without adding this additional code to Package.appxmanifest, CommunityToolkit.Maui causes all Windows apps to crash on launch.

This PR adds Options.ShouldEnableSnackbarOnWidows to allow users to opt-in to enabling Snackbar on Windows, ensuring that they have added the additional required code to Package.appxmanifest.

var builder = MauiApp.CreateBuilder()
  .UseMauiCommunityToolkit(options =>
  {
    options.SetShouldEnableSnackbarOnWindows(true);
  })

[bijington]

Good work @brminnick I think we should either use the value parameter or remove it though.

[brminnick]

Thanks Shaun! Good catch. Updated ✅

[brminnick]

Thanks Shaun + Vlad! I think I'm happy with the implementation now.

We've added enough fail-safes to the code to assist future devs who haven't yet seen the platform-specific implementation docs for Snackbar on Windows. Hopefully throwing these exceptions at runtime will help folks find the docs instead of opening a new Issue on the repo.

And if they do open an issue for one of these InvalidOperationExceptions, we can just copy/paste the link from the stack trace, point them to the docs and close the issue.