Tree: add node-contextmenu event for tree node (#9678)

packages/tree/src/tree-node.vue

@@ -2,6 +2,7 @@
   <div
     class="el-tree-node"
     @click.stop="handleClick"
+    @contextmenu="($event) => this.handleContextMenu($event)"
     v-show="node.visible"
     :class="{
       'is-expanded': expanded,
@@ -164,6 +165,10 @@
         this.tree.$emit('node-click', this.node.data, this.node, this);
       },
 
+      handleContextMenu(event) {
+        this.tree.$emit('node-contextmenu', event, this.node.data, this.node, this);
+      },
+
       handleExpandIconClick() {
         if (this.node.isLeaf) return;
         if (this.expanded) {