fix: add split button token

packages/semi-foundation/button/splitButtonGroup.scss

@@ -12,13 +12,13 @@ $icon: #{$module}-with-icon;
         margin-right: 1px;
 
         &-first {
-            border-top-left-radius: var(--semi-border-radius-small);
-            border-bottom-left-radius: var(--semi-border-radius-small);
+            border-top-left-radius: $radius-splitButtonGroup_first_topLeft;
+            border-bottom-left-radius: $radius-splitButtonGroup_first_bottomLeft;
         }
 
         &-last {
-            border-top-right-radius: var(--semi-border-radius-small);
-            border-bottom-right-radius: var(--semi-border-radius-small);
+            border-top-right-radius: $radius-splitButtonGroup_last_topRight;
+            border-bottom-right-radius: $radius-splitButtonGroup_last_bottomRight;
             margin-right: unset;
         }
     }

packages/semi-foundation/button/variables.scss

@@ -160,3 +160,9 @@ $width-button-border: $border-thickness; // 按钮描边宽度
 $radius-button: var(--semi-border-radius-small); // 按钮圆角大小
 $radius-button_group: $radius-button; // 按钮组圆角大小
 $width-button-outline: 2px; // 按钮轮廓宽度
+
+
+$radius-splitButtonGroup_first_topLeft: var(--semi-border-radius-small); // split button 左上圆角
+$radius-splitButtonGroup_first_bottomLeft:var(--semi-border-radius-small); // split button 左下圆角
+$radius-splitButtonGroup_last_topRight:var(--semi-border-radius-small); // split button 右上圆角
+$radius-splitButtonGroup_last_bottomRight:var(--semi-border-radius-small); // split button 右下圆角