ButtonGroup: fix Dropdown style in ButtonGroup(#12815) (#12819)

* [Button]Support Dropdown in ButtonGroup(#12815)

* [Button]Support Dropdown in ButtonGroup(#12815)

* [Button]Support Dropdown in ButtonGroup(#12815)

Author: bluejfox

packages/theme-chalk/src/button.scss

@@ -186,7 +186,7 @@
   display: inline-block;
   vertical-align: middle;
 
-  & .el-button {
+  & > .el-button {
     float: left;
     position: relative;
     & + .el-button {
@@ -232,6 +232,14 @@
       z-index: 1;
     }
   }
+  
+  & > .el-dropdown {
+    & > .el-button {
+      border-top-left-radius: 0;
+      border-bottom-left-radius: 0;
+      border-left-color: rgba($--color-white, 0.5);
+    }
+  }
 
   @each $type in (primary, success, warning, danger, info) {
     .el-button--#{$type} {