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	99.7 kB (+1 B)	37.8 kB (+1 B)	34.2 kB (+44 B)
vue.global.prod.js	158 kB (+1 B)	57.7 kB (+2 B)	51.4 kB (+28 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	47 kB (+1 B)	18.3 kB	16.8 kB (+45 B)
createApp	55.1 kB (+1 B)	21.3 kB (-2 B)	19.4 kB (-4 B)
createSSRApp	59.1 kB (+1 B)	23 kB (-1 B)	20.9 kB (+1 B)
defineCustomElement	59.9 kB (+1 B)	22.9 kB (-2 B)	20.8 kB
overall	68.9 kB (+1 B)	26.4 kB	24 kB (-14 B)

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commit: 3d986d2