Add basic benchmarks for Is and Class selectors.

tests/Avalonia.Benchmarks/Styling/SelectorBenchmark.cs

@@ -0,0 +1,52 @@
+using Avalonia.Controls;
+using Avalonia.Styling;
+using BenchmarkDotNet.Attributes;
+
+namespace Avalonia.Benchmarks.Styling
+{
+    [MemoryDiagnoser]
+    public class SelectorBenchmark
+    {
+        private readonly Control _notMatchingControl;
+        private readonly Calendar _matchingControl;
+        private readonly Selector _isCalendarSelector;
+        private readonly Selector _classSelector;
+
+        public SelectorBenchmark()
+        {
+            _notMatchingControl = new Control();
+            _matchingControl = new Calendar();
+
+            const string className = "selector-class";
+
+            _matchingControl.Classes.Add(className);
+
+            _isCalendarSelector = Selectors.Is<Calendar>(null);
+            _classSelector = Selectors.Class(null, className);
+        }
+
+        [Benchmark]
+        public SelectorMatch IsSelector_NoMatch()
+        {
+            return _isCalendarSelector.Match(_notMatchingControl);
+        }
+
+        [Benchmark]
+        public SelectorMatch IsSelector_Match()
+        {
+            return _isCalendarSelector.Match(_matchingControl);
+        }
+
+        [Benchmark]
+        public SelectorMatch ClassSelector_NoMatch()
+        {
+            return _classSelector.Match(_notMatchingControl);
+        }
+
+        [Benchmark]
+        public SelectorMatch ClassSelector_Match()
+        {
+            return _classSelector.Match(_matchingControl);
+        }
+    }
+}