feat: add button related resources.

src/Semi.Avalonia.Demo/Pages/ButtonDemo.axaml

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+<UserControl 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"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="Semi.Avalonia.Demo.Pages.ButtonDemo">
+    <StackPanel Spacing="20" Margin="20" HorizontalAlignment="Left">
+        <Button>Primary</Button>
+        <Button Classes="Large">Primary</Button>
+        <Button Classes="Small">Primary</Button>
+        <Button Classes="Primary">Secondary</Button>
+    </StackPanel>
+</UserControl>

src/Semi.Avalonia.Demo/Pages/ButtonDemo.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Semi.Avalonia.Demo.Pages;
+
+public partial class ButtonDemo : UserControl
+{
+    public ButtonDemo()
+    {
+        InitializeComponent();
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}