dx(reactivity): warn on nested readonly ref update during unwrapping

[jh-leong]

related to #12138

1. Warn instead of throwing an error when updating a nested readonly ref, keeping consistency with the readonly API.
2. Improved error message for better clarity.

Before: TypeError: 'set' on proxy: trap returned falsish for property 'foo'
After: [Vue warn] Set operation on key "foo" failed: target is readonly.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB (+1 B)	38.1 kB (+2 B)	34.3 kB (+65 B)
vue.global.prod.js	160 kB (+1 B)	58 kB (+1 B)	51.5 kB (-70 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49.1 kB (+1 B)	18.9 kB	17.3 kB (+1 B)
createApp	55.7 kB (+1 B)	21.4 kB (-1 B)	19.6 kB
createSSRApp	59.7 kB (+1 B)	23.1 kB	21.1 kB (+42 B)
defineCustomElement	60.4 kB (+1 B)	23 kB	20.9 kB (-8 B)
overall	69.4 kB (+1 B)	26.5 kB	24.1 kB (+73 B)

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commit: 0453906