feat: add avalonia GridSplitter style.

Author: NaBian

src/Avalonia/HandyControl_Avalonia/Themes/Styles/GridSplitter.axaml

@@ -0,0 +1,30 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+    <ControlTheme x:Key="{x:Type GridSplitter}"
+                  TargetType="GridSplitter">
+        <Setter Property="Focusable"
+                Value="True" />
+        <Setter Property="MinWidth"
+                Value="1" />
+        <Setter Property="MinHeight"
+                Value="1" />
+        <Setter Property="Background"
+                Value="{DynamicResource BorderBrush}" />
+        <Setter Property="PreviewContent">
+            <Template>
+                <Rectangle Fill="{DynamicResource DarkOpacityBrush}" />
+            </Template>
+        </Setter>
+        <Setter Property="Template">
+            <ControlTemplate>
+                <Border Background="{TemplateBinding Background}"
+                        BorderBrush="{TemplateBinding BorderBrush}"
+                        BorderThickness="{TemplateBinding BorderThickness}"
+                        CornerRadius="{TemplateBinding CornerRadius}" />
+            </ControlTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -25,6 +25,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/OverlayPopupHost.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PathIcon.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Separator.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/GridSplitter.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ToolTip.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/TextBlock.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ContentControl.axaml" />