fix(customElement): properly get native attribute value

[edison1105]

close #12124

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB (+67 B)	38.1 kB (+21 B)	34.2 kB (+30 B)
vue.global.prod.js	160 kB (+67 B)	58 kB (+21 B)	51.6 kB (+42 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49.1 kB	18.9 kB	17.3 kB
createApp	55.7 kB	21.4 kB	19.6 kB
createSSRApp	59.7 kB	23.1 kB	21 kB
defineCustomElement	60.5 kB (+67 B)	23 kB (+20 B)	21 kB (+48 B)
overall	69.4 kB	26.5 kB	24.1 kB

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commit: 3b7fd30

[lennartbuit]

Its not just tagName, its anything from at least Element that is problematic. Like, I can also override hasAttribute.

So can we instead warn when you define a property the object already has?