BREAKING CHANGE: watch APIs now default to use `flush: 'pre'` instead of
`flush: 'post'`.
- This change affects `watch`, `watchEffect`, the `watch` component
option, and `this.$watch`.
- As pointed out by @skirtles-code in
[this comment](https://github.com/vuejs/vue-next/issues/1706#issuecomment-666258948),
Vue 2's watch behavior is pre-flush, and the ecosystem has many uses
of watch that assumes the pre-flush behavior. Defaulting to post-flush
can result in unnecessary re-renders without the users being aware of
it.
- With this change, watchers need to specify `{ flush: 'post' }` via
options to trigger callback after Vue render updates. Note that
specifying `{ flush: 'post' }` will also defer `watchEffect`'s
initial run to wait for the component's initial render.
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