feat: add button group.

demo/Ursa.Demo/Pages/ButtonGroupDemo.axaml

@@ -0,0 +1,19 @@
+<UserControl
+    x:Class="Ursa.Demo.Pages.ButtonGroupDemo"
+    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"
+    xmlns:vm="clr-namespace:Ursa.Demo.ViewModels;assembly=Ursa.Demo"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    x:CompileBindings="True"
+    x:DataType="vm:ButtonGroupViewModel"
+    mc:Ignorable="d">
+
+    <StackPanel Margin="20" Spacing="20">
+        <u:ButtonGroup Classes="Primary Solid" ItemsSource="{Binding Items}" />
+        <u:ButtonGroup Classes="Primary" ItemsSource="{Binding Items}" />
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Pages/ButtonGroupDemo.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Pages;
+
+public partial class ButtonGroupDemo : UserControl
+{
+    public ButtonGroupDemo()
+    {
+        InitializeComponent();
+        this.DataContext = new ButtonGroupViewModel();
+    }
+}