Add HasClass extension method

This is a common enough check on web elements that it deserves an easy built-in method.

Author: kzu

src/Tests/HtmlTests.cs

@@ -98,5 +98,19 @@ public void HtmlSettings()
         Assert.Equal("Wikipedia", central?.Value);
     }
 
+    [Fact]
+    public void HasCssClass()
+    {
+        // We wrap the doc with hml>body automatically, so we need to select the div
+        var div = HtmlDocument.Parse(
+            """
+            <div class="foo bar baz" />
+            """)
+            .CssSelectElement("div");
+
+        Assert.True(div.HasClass("foo"));
+        Assert.True(div.HasClass("bar"));
+    }
+
     string File(string path) => new Uri("file://" + new FileInfo(path).FullName).AbsoluteUri;
 }

src/Web/CssSelectorExtensions.cs

@@ -49,6 +49,15 @@ public static IEnumerable<XElement> CssSelectElements(this XNode? node, string e
         return node.XPathSelectElements(xpath, new CssContext());
     }
 
+    /// <summary>
+    /// Determines whether the <see cref="XNode"/> has the specified class.
+    /// </summary>
+    public static bool HasClass(this XNode? node, string className)
+    {
+        var element = node as XElement;
+        return element?.Attribute("class")?.Value.Split(' ').Contains(className) ?? false;
+    }
+
     // The custom context allows resolving the fn:sum which properly implements the XPath 2.0 fn:sum
     // see https://www.w3.org/TR/xquery-operators/#func-sum.
     class CssContext : XsltContext