feat(DatetimePicker): support picker slots (#7645)

src/datetime-picker/test/__snapshots__/datetime-picker.spec.js.snap

@@ -1,5 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`should render title slot correctly 1`] = `<div class="van-picker__toolbar"><button type="button" class="van-picker__cancel">取消</button>Custom title<button type="button" class="van-picker__confirm">确认</button></div>`;
+
 exports[`time type 1`] = `
 <div class="van-picker van-datetime-picker">
   <div class="van-picker__toolbar"><button type="button" class="van-picker__cancel">取消</button><button type="button" class="van-picker__confirm">确认</button></div>

src/datetime-picker/test/datetime-picker.spec.js

@@ -35,3 +35,16 @@ test('getPicker method', () => {
   const wrapper = mount(DatetimePicker);
   expect(wrapper.vm.getPicker()).toBeTruthy();
 });
+
+test('should render title slot correctly', () => {
+  const wrapper = mount(DatetimePicker, {
+    propsData: {
+      showToolbar: true,
+    },
+    scopedSlots: {
+      title: () => 'Custom title',
+    },
+  });
+
+  expect(wrapper.find('.van-picker__toolbar')).toMatchSnapshot();
+});