fix(hmr):using customElements after hmr will trigger an error

[ysy945]

• problem link
• when I use customElements it's ok. but after changing file will trigger hmr.,it will be rerun window.customElements.define. so there is an error will be caused.
  
• I think just need to set a cache to check that customElement has been registered or not.

[ysy945]

@sxzz Excuse me, do you have time to review this pr. thanks

[edison1105]

I think this is outside the scope of vue. the developers should ensure that custom elements are not registered many times.
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