[Components] Enable a root component to be the entry point for component based apps

[javiercn]

This is a tracking issue for enabling applications that only use components end to end. Here we will discuss and track the work that needs to be done to enable this scenario:

• Support DOCTYPE declarations.
• Support a way to mark a component as a "root component" so that it can be used as the container for the entire app.
• Figure out a way to turn prerrendering on and off selectively.
• Figure out what component equivalents we need for things like the Environment tag helper, '~/' etc.
• Figure out specific integrations with endpoint routing.

[Andrzej-W]

The concept of "root component" is very nice. Please remember about SEO. Prerendered HTML page has to have correct title, description, metatags. More about this here: https://github.com/aspnet/Blazor/issues/1311#issuecomment-413046895