Add Windows Forms TaskDialog API sample

[RussKie]

Summary

Add a sample for Windows Forms TaskDialog API, that was added in .NET 5.0 in dotnet/winforms#1133.

Credits: this sample was authored by @kpreisser in https://github.com/kpreisser/TaskDialogDemo, the author of Task Dialog for Windows Forms.

[RussKie]

/cc: @OliaG @merriemcgaw @Olina-Zhang

[adegeo]

Thanks for adding this here! This sample should be split into two, a csharp one and a vb one. Basically, add a readme in the root directory for each sample, it should contain a metadata header so that the sample shows up in the samples browser. Here is an example of the header: https://raw.githubusercontent.com/dotnet/samples/master/mef/simple-calculator/vb/readme.md

Which looks like this:

---
languages:
- vb
products:
- dotnet-core
- windows
page_type: sample
name: "MEF Sample: Simple Calculator (Visual Basic)"
urlFragment: "simple-calculator-vb"
description: "A .NET Core console application that contains the example method from Simple Calculator MEF Application in Visual Basic."
---

[adegeo]

Changes requested based on the previous comment.

[RussKie]

Updated, please have a look

[adegeo]

Thanks! Which preview of .NET 5 is required?

[RussKie]

.NET 5.0 Preview 4

[adegeo]

Final changes and we're good-to-go. Thanks a bunch for helping out!

[RussKie]

done

[adegeo]

I'm going to hold off on merging this. I have a PR that enables our CI testing and I need to update it for .NET 5, so this PR is a great test case. Is that OK? Let me know. Thanks!

[RussKie]

No worries. No rush on my part. Thank you