chore: add issue template

Author: yyx990803

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+IMPORTANT: Please use the following link to create a new issue:
+
+  https://new-issue.vuejs.org/?repo=vuejs/vue-test-utils
+
+If your issue was not created using the app above, it will be closed immediately.
+-->