Add some more analyzer tests

CliFx.Analyzers/ParameterMustBeSingleIfNonRequiredAnalyzer.cs

@@ -0,0 +1,60 @@
+using System.Linq;
+using CliFx.Analyzers.ObjectModel;
+using CliFx.Analyzers.Utils.Extensions;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public class ParameterMustBeSingleIfNonRequiredAnalyzer : AnalyzerBase
+{
+    public ParameterMustBeSingleIfNonRequiredAnalyzer()
+        : base(
+            "Parameters marked as non-required are limited to one per command",
+            "This parameter is non-required so it must be the only such parameter in the command.")
+    {
+    }
+
+    private void Analyze(
+        SyntaxNodeAnalysisContext context,
+        PropertyDeclarationSyntax propertyDeclaration,
+        IPropertySymbol property)
+    {
+        if (property.ContainingType is null)
+            return;
+
+        var parameter = CommandParameterSymbol.TryResolve(property);
+        if (parameter is null)
+            return;
+
+        if (parameter.IsRequired != false)
+            return;
+
+        var otherProperties = property
+            .ContainingType
+            .GetMembers()
+            .OfType<IPropertySymbol>()
+            .Where(m => !m.Equals(property, SymbolEqualityComparer.Default))
+            .ToArray();
+
+        foreach (var otherProperty in otherProperties)
+        {
+            var otherParameter = CommandParameterSymbol.TryResolve(otherProperty);
+            if (otherParameter is null)
+                continue;
+
+            if (otherParameter.IsRequired == false)
+            {
+                context.ReportDiagnostic(CreateDiagnostic(propertyDeclaration.GetLocation()));
+            }
+        }
+    }
+
+    public override void Initialize(AnalysisContext context)
+    {
+        base.Initialize(context);
+        context.HandlePropertyDeclaration(Analyze);
+    }
+}