Set default lazy option to true in watch api

[takumi-n]

This pull request fixes that watch function runs when a source variable has been defined.

https://vue-composition-api-rfc.netlify.com/api.html#watch

The watch API is the exact equivalent of the 2.x this.$watch (and the corresponding watch option). watch requires watching a specific data source, and applies side effects in a separate callback function. It also is lazy by default - i.e. the callback is only called when the watched source has changed.

As stated in Vue Composition API documents, the callback function passed to watch should run only when the source has changed.

vuejs/composition-api always sets lazy field of WatcherOption to false by default,
https://github.com/vuejs/composition-api/blob/master/src/apis/watch.ts#L60

but it should be true in watch

Resolve #266

[antfu]

Hi, I think the lazy option is replaced by immediate and default to false. I have already made PR #300 addressing that.

[takumi-n]

@antfu
Hi, I have overlooked your PR.
I think that vuejs/composition-api keeps lazy for two reasons.

1. As you have said, switching from lazy to immediate breaks compatibility.
2. vuejs/composition-api could differ from RFC and Vue.js 3 because lazy is stated in RFC

[jacekkarczmarczyk]

https://github.com/vuejs/vue-next/blob/master/CHANGELOG.md#breaking-changes-5 @takumi-n

[takumi-n]

@jacekkarczmarczyk
Thanks. I didn't know that at all.

I think #300 is better.