feat: initialize.

demo/Ursa.Demo/Pages/NumericUpDownDemo.axaml

@@ -0,0 +1,14 @@
+<UserControl
+    x:Class="Ursa.Demo.Pages.NumericUpDownDemo"
+    xmlns="https://github.com/avaloniaui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    xmlns:u="https://irihi.tech/ursa"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mc:Ignorable="d">
+    <StackPanel>
+        <u:IntUpDown />
+    </StackPanel>
+</UserControl>