fix: add support when using @vue/compat aka migration build

[renatodeleao]

Users migrating vue2 apps to vue3 usually start with this package^1 if their app is non-trivial. The migration build is usually set with MODE: 2 for having maximum backwards compatibility, but since this component is written in vue3 format we need to let the Vue compiler know.

Alternative: Manually patching

Users can work around this by manually patching the exported object, but less experienced users might not be aware of it when they just see a runtime error in the console.

import VueCountdown from '@chenfengyuan/vue-countdown'
VueCountdown.compatConfig = { MODE: 3 }

[Scrum]

@fengyuanchen Hi, are there any plans to merge this? Let me know if you need help maintaining this package, I will be glad