fix: fix multiple selection, TreeSelect arrow width is not uniform (#983)

packages/semi-foundation/treeSelect/treeSelect.scss

@@ -239,6 +239,7 @@ $module: #{$prefix}-tree-select;
         // right: 0;
         display: inline-flex;
         align-items: center;
+        flex-shrink: 0;
         height: 100%;
         justify-content: center;
         width: $width-treeSelect_arrow;
@@ -265,6 +266,7 @@ $module: #{$prefix}-tree-select;
         display: inline-flex;
         align-items: center;
         height: 100%;
+        flex-shrink: 0;
         justify-content: center;
         width: $width-treeSelect_arrow;
         color: $color-treeSelect_default-icon-default;