test(watch): watching a reactive array

[skirtles-code]

These tests were inspired by the changes in #9417. I don't think the changes proposed there can be merged as they would break backwards compatibility, but I found it interesting that they didn't cause the existing tests to fail.

It seems we have relatively few tests for watch(reactiveArray, ...), and in particular how it differs from watch(plainArray, ...). I experimented locally and found I was able to introduce various bugs without triggering any test failures.

Here I've aimed to test 3 ways that reactive arrays behave differently from plain arrays when passed as a source to watch:

1. If the reactive array contains a function, it should not be invoked.
2. If the reactive array contains a ref, it should not be unwrapped in the newValue or oldValue passed to the callback.
3. A reactive array should be deep: true by default.
4. A reactive array with an explicit deep: false should be watched one level deep, starting from the array itself.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	90.7 kB	34.5 kB	31.1 kB
vue.global.prod.js	148 kB	53.7 kB	48 kB

Usages

Name	Size	Gzip	Brotli
createApp	50.8 kB	19.9 kB	18.1 kB
createSSRApp	54.1 kB	21.2 kB	19.3 kB
defineCustomElement	53.1 kB	20.6 kB	18.8 kB
overall	64.5 kB	24.9 kB	22.5 kB