nested component styles will discard when using defineCustomElements

[luoliwoshang]

#34
Vue cannot currently render styles of nested components in custom element，Because Vue does not register the styles of nested components in a custom element into the shadowdom of a custom element。

nested component styles discard The author of Vue mentioned

this could be a use case where the library author don't want to pre-determine the tags the elements are registered as
If the child components are compiled under custom elements mode, it's probably better to register them and use them as custom elements.

That is to say, if we want to use the nested component in custom element and load its style correctly, we need to develop the nested component as a custom element.