fix(customElement): preserve appContext during hmr reload

[edison1105]

close #12453

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB (+70 B)	38 kB (+18 B)	34.2 kB (-22 B)
vue.global.prod.js	158 kB (+70 B)	57.8 kB (+17 B)	51.5 kB (+30 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	47.2 kB	18.3 kB	16.8 kB
createApp	55.2 kB	21.3 kB	19.5 kB
createSSRApp	59.3 kB	23.1 kB	21 kB
defineCustomElement	60.1 kB (+72 B)	22.9 kB (+14 B)	20.9 kB (+17 B)
overall	69.1 kB	26.5 kB	24.1 kB

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commit: b07d1db