feat(Step): add 'CurrentStepIndex' to obtain the real-time value of _currentStepIndex

Author: momijijin

src/BootstrapBlazor/Components/Step/Step.razor.cs

@@ -53,6 +53,11 @@ public partial class Step
 
     private int _currentStepIndex;
 
+    /// <summary>
+    /// 获得当前步骤索引（只读）
+    /// </summary>
+    public int CurrentStepIndex => _currentStepIndex;
+
     /// <summary>
     /// 获得 组件样式字符串
     /// </summary>