This commit reorganizes the Angular code a bit, and moves signals into a
newly defined `@angular/core/primitives` location. This will be used inside
g3 to allow non-Angular targets to depend on the signals core without
incurring a dependency on the whole framework.
PR Close#51986
This commit adds the basic sketch for the implementation of `fromObservable`
and `fromSignal`, the two basic primitives which form the RxJS interop layer
with signals.
PR Close#49154