[2.x] Restore `router.resolveComponent()` method

[pascalbaljet]

First of all, thanks for all the great work on v2! ❤️

In v1, the resolveComponent method was available on the router instance. This PR restores that for v2.

I am using this in my modal package to resolve the modal component, similarly to resolving a page. Without having this method publicly available, I must duplicate/reference the resolveComponent method in the main app.js file. That would be a shame, as it's so clean and nice in v1 :)