Add VB example to Xaml designer extensibility

[Nukepayload2]

Issue: #3602

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[PRMerger15]

@Nukepayload2 : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[mgoertz-msft]

Looks good to me. Thank you for contributing! :-)

[mgoertz-msft]

Would you mind fixing the C# sample too? I'm pretty sure this was meant to say this:
if (type != null && type.IsSubclassOf(buttonType)) or shorter to match your change:
if (type?.IsSubclassOf(buttonType) == true)

[Youssef1313]

I suggest you change this line
AttributeTableBuilder builder = new AttributeTableBuilder();
with this one:
var builder = new AttributeTableBuilder();

Because C# Coding Conventions says:

Use implicit typing for local variables when the type of the variable is obvious from the right side of the assignment, or when the precise type is not important.

[lutzroeder]

#sign-off

[PRMerger10]

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[Jak-MS]

@lutzroeder - I'll merge this, but I can't verify that the links in line 44 are working correctly. Can you check them after the changes are live? Thanks.