WatchCallback returns `void`, not `any`

[rotu]

This matches the documentation and better expresses that the return value is (mostly) ignored.

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Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	37.9 kB	34.2 kB
vue.global.prod.js	158 kB	57.8 kB	51.4 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB	18.3 kB	16.7 kB
createApp	54.6 kB	21.3 kB	19.4 kB
createSSRApp	58.7 kB	23 kB	20.9 kB
defineCustomElement	59.4 kB	22.8 kB	20.8 kB
overall	68.4 kB	26.4 kB	24 kB

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commit: 09e84ef

[rotu]

Maybe both the docs and the return type of WatchCallback should be changed to undefined | Promise<undefined>?

void provides very little type-checking since a function returning void may return any value.

Relevant discussion: https://github.com/microsoft/TypeScript/issues/42709

[skirtles-code]

I'm not clear what the correct type should be, but a couple of observations:

1. This seems a similar idea to #5914. In that case the decision was to use any.
2. The return types for WatchCallback and WatchEffect are currently different (any and void respectively). I would've expected them to be the same.