[workload] include Design/Microsoft.Maui.Controls.Xaml.DesignTools.dll

[jonathanpeppers]

Description of Change

Fixes: #1699

This adds the missing "design" assembly in our various
Microsoft.Maui.Controls.Ref.* packs, such as:

> 7z l .\artifacts\Microsoft.Maui.Controls.Ref.android.0.0.1-alpha1.nupkg
   Date      Time    Attr         Size   Compressed  Name
------------------- ----- ------------ ------------  ------------------------
2021-07-16 11:05:26 .....          529          296  _rels\.rels
2021-07-16 11:05:26 .....          799          421  Microsoft.Maui.Controls.Ref.android.nuspec
2021-07-16 15:38:00 .....        10752         3894  ref\net6.0-android30.0\Microsoft.Maui.Controls.Compatibility.Android.FormsViewGroup.dll
2021-07-16 15:37:56 .....         2259         2258  ref\net6.0-android30.0\Microsoft.Maui.Controls.Compatibility.Android.FormsViewGroup.aar
2021-07-16 15:38:22 .....         7010         5676  ref\net6.0-android30.0\Microsoft.Maui.Controls.Compatibility.aar
2021-07-16 15:38:22 .....       361472       111051  ref\net6.0-android30.0\Microsoft.Maui.Controls.Compatibility.dll
2021-07-16 15:38:14 .....       676352       216556  ref\net6.0-android30.0\Microsoft.Maui.Controls.dll
2021-07-16 15:38:18 .....       215552        62234  ref\net6.0-android30.0\Microsoft.Maui.Controls.Xaml.dll
2021-07-16 16:05:14 .....        11264         2657  ref\net6.0-android30.0\Design\Microsoft.Maui.Controls.DesignTools.dll
2021-07-16 15:37:56 .....         4608         1544  ref\net6.0-android30.0\Design\Microsoft.Maui.Controls.Xaml.DesignTools.dll
2021-07-16 16:05:26 .....         1581          290  data\FrameworkList.xml
2021-07-16 11:05:26 .....          591          223  [Content_Types].xml
2021-07-16 11:05:26 .....          662          396  package\services\metadata\core-properties\b8f1a5e1756847dca0c02a1e31dd6a34.psmdcp
------------------- ----- ------------ ------------  ------------------------
2021-07-16 16:05:26            1293431       407496  13 files

I don't know how to verify this location is correct, but it seems to
mirror what we had in .NET MAUI Preview 5:

https://www.nuget.org/api/v2/package/Microsoft.Maui.Controls/6.0.100-preview.5.794

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

No