[FR] Make components subclassable without template

### Which @angular/* package(s) are relevant/related to the feature request?

core

### Description

Angular lately added support for subclassing a component. It is thus now possible for a downstream-app to override a component's base-implementation in a lib. According to the docs I read, all you have to do is declare the same `selector`. Unfortunately, though, the subclass requires `template` or `templateUrl` -- without it, I get this error:
```
error NG2001: component is missing a template
```
My app-code doesn't have access to the lib's template-file (declared there via `templateUrl`) and IMHO I should not be forced to re-declare the template, if I do not want to replace it (but inherit it the way it is in the super-component).

The component-class contains the UI-logic. It should be easily possible for a downstream-lib/-app to customize this logic, i.e. override protected/public methods, without touching the UI (i.e. without re-declaring a template).

### Proposed solution

Make the `@Component(...)`-annotation's `template` and `templateUrl` optional for component-subclasses. A component-subclass should easily be able to override UI-logic without providing its own template.

### Alternatives considered

We tried to extract the UI-logic from the component into a stateful service which is associated 1-to-1 with its component. This, however, doesn't work either -- hence, I opened this other feature-request: https://github.com/angular/angular/issues/47899

Start in Angular's core component inheritance and compilation behavior around the reported NG2001 error. Reproduce a subclassed component without template or templateUrl, then trace how the superclass template is handled. Done means a component subclass can override UI logic without redeclaring the inherited template, with coverage for the supported behavior.

Written by the indexing model from the issue text.