feat: add a default component for nested routes

[deckar01]

Fixes #2105

Allows nested child routes to render inside routes without a component. Previously this prevented the child route's component from rendering without any warning. Now the parent's component will default to a PassThrough component that provides the <router-view> the child needs to render in.

[deckar01]

This needs docs and more test coverage, but I thought I should get some feedback before investing any more time into this.

[posva]

I appreciate the initiative but this is not something we want to add, at least yet. That's also why the issue was closed.

A note about the implementation you made: the component should be functional and there should be a render function instead of a template

[deckar01-bot]

I will just fork then. Thanks for the feedback.