Downgrade target Roslyn version in analyzers

Closes #135

CliFx.Analyzers/Utils/Extensions/RoslynExtensions.cs

@@ -34,6 +34,17 @@ internal static class RoslynExtensions
         .FirstOrDefault(i => i.ConstructedFrom.SpecialType == SpecialType.System_Collections_Generic_IEnumerable_T)?
         .TypeArguments[0];
 
+    // Detect if the property is required through roundabout means so as to not have to take dependency
+    // on higher versions of the C# compiler.
+    public static bool IsRequired(this IPropertySymbol property) => property
+        // Can't rely on the RequiredMemberAttribute because it's generated by the compiler, not added by the user,
+        // so we have to check for the presence of the `required` modifier in the syntax tree instead.
+        .DeclaringSyntaxReferences
+        .Select(r => r.GetSyntax())
+        .OfType<PropertyDeclarationSyntax>()
+        .SelectMany(p => p.Modifiers)
+        .Any(m => m.IsKind((SyntaxKind)8447));
+
     public static bool IsAssignable(this Compilation compilation, ITypeSymbol source, ITypeSymbol destination) =>
         compilation.ClassifyConversion(source, destination).Exists;