Waf DotNetPad

• Update target framework to 4.7.2
• Update to Microsoft.CodeAnalysis 3.7.0
• Enable C# 9 preview support
• Support NullableAttributes and Range, Indices
• Improve .NET Framework version detection
• Replace png icons with xaml icons
• Add NullableReferenceTypes sample
• Add SwitchExpression sample
• Add CS9Preview sample