fix(types/toRef): ensure compatibility for ShallowRef unions in TS <=5.3

[jh-leong]

close #12986

This change may slightly reduce type readability:

declare const v: ShallowRef<number> | number
const _ = toRef(v)
// Before: Ref<number, number> | ShallowRef<number>
// After:  Ref<number, number> | SafeShallowRef<ShallowRef<number>>

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	38.1 kB	34.3 kB
vue.global.prod.js	158 kB	58 kB	51.6 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.5 kB	18.2 kB	16.7 kB
createApp	54.6 kB	21.3 kB	19.4 kB
createSSRApp	58.8 kB	23 kB	21 kB
defineCustomElement	59.4 kB	22.9 kB	20.8 kB
overall	68.6 kB	26.4 kB	24.1 kB

[jh-leong]

/ecosystem-ci run

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commit: 122f05a

[vue-bot]

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