feat(reactivity): store value cache on CustomRefs impls

[alexzhang1030]

VueDevtools will access refs value to display on the devtools panel.

ref has its own _value, but customRefs are not (like customRef() or toRef(). Implementing a value cache is necessary to prevent potential side-effects or performance issues when VueDevtools reads these values.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	98.4 kB (+8.18 kB)	37.3 kB (+2.72 kB)	33.6 kB (+2.39 kB)
vue.global.prod.js	156 kB (+9.15 kB)	57 kB (+2.98 kB)	50.7 kB (+2.58 kB)

Usages

Name	Size	Gzip	Brotli
createApp	53.9 kB (+4.29 kB)	20.9 kB (+1.43 kB)	19.1 kB (+1.31 kB)
createSSRApp	57.9 kB (+4.64 kB)	22.5 kB (+1.56 kB)	20.5 kB (+1.41 kB)
defineCustomElement	58.5 kB (+6.61 kB)	22.4 kB (+2.15 kB)	20.4 kB (+1.9 kB)
overall	67.5 kB (+4.34 kB)	25.9 kB (+1.45 kB)	23.6 kB (+1.32 kB)