fix(watch): watchEffect clean-up with SSR

[skirtles-code]

close #11956 see #11956 (comment)

Background: #11956 (comment)

#11884 removed internal support for using once: true with watchEffect. While this was never part of the public API, it was being used internally to perform clean-up during SSR.

This potentially leads to watchEffect leaking during SSR, though I believe this would only be a problem in practice if it was watching data that was retained across requests.

Example:

• Playground - 3.5.10
• Playground - this PR

Rather than adding support back into @vue/reactivity, I've tried to implement a fix that will only be included for SSR builds.

I call the stop function immediately after creating the watcher. In the cases we care about, the watcher should have already run by this point.

I've made a big assumption: watchEffect with flush: 'post' should not run during SSR. I don't know whether this assumption is valid, but the approach I've taken here won't work if that isn't correct.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB	38.1 kB	34.3 kB
vue.global.prod.js	160 kB	58 kB	51.5 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49 kB	18.9 kB	17.2 kB
createApp	55.6 kB (+18 B)	21.4 kB (+17 B)	19.5 kB (-70 B)
createSSRApp	59.7 kB (+18 B)	23.1 kB (+13 B)	21 kB (+18 B)
defineCustomElement	60.4 kB (+18 B)	23 kB (+20 B)	21 kB (+81 B)
overall	69.4 kB (+18 B)	26.5 kB (+18 B)	24.1 kB (+30 B)

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commit: bfe33d8

[edison1105]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

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