refactor(ivy): make return value of define(Component|Directive|Pipe|Injector|Injectable) private (#23371)

Ivy definition looks something like this:

```
class MyService {
  static ngInjectableDef = defineInjectable({
    …
  });
}
```

Here the argument to `defineInjectable` is well known public contract which needs
to be honored in backward compatible way between versions. The type of the
return value of `defineInjectable` on the other hand is private and can change
shape drastically between versions without effecting backwards compatibility of
libraries publish to NPM. To our users it is effectively an `OpaqueToken`.

By prefixing the type with `ɵ` we are communicating the the outside world that
the value is not public API and is subject to change without backward compatibility.

PR Close #23371

packages/core/test/di/r3_injector_spec.ts

@@ -11,7 +11,7 @@ import {InjectionToken} from '../../src/di/injection_token';
 import {INJECTOR, Injector, inject} from '../../src/di/injector';
 import {R3Injector, createInjector} from '../../src/di/r3_injector';
 
-describe('InjectorDef-based createInjector()', () => {
+describe('ɵInjectorDef-based createInjector()', () => {
   class CircularA {
     static ngInjectableDef = defineInjectable({
       providedIn: null,