[improvement] mixins typescript (#3690)

src/mixins/slots.ts

@@ -0,0 +1,20 @@
+/**
+ * Use scopedSlots in Vue 2.6+
+ * downgrade to slots in lower version
+ */
+import Vue from 'vue';
+
+export const SlotsMixin = Vue.extend({
+  methods: {
+    slots(name = 'default', props: any) {
+      const { $slots, $scopedSlots } = this;
+      const scopedSlot = $scopedSlots[name];
+
+      if (scopedSlot) {
+        return scopedSlot(props);
+      }
+
+      return $slots[name];
+    }
+  }
+});