feat(core): add Angular Signals to the public API

[alxhub]

For context, see this Github Discussion on our experiments with fine-grained reactivity in Angular

This commit exposes signal, computed, effect and various helpers from the @angular/core entrypoint.

These APIs are marked as @developerPreview and are still prototypes in active development. Their final shapes will be subject to our internal design reviews as well as one or more community RFCs. We're exporting them now to allow for experimentation using 16.0.0 next and RC releases.

[dylhunn]

reviewed-for: fw-core, public-api

[AndrewKushnir]

Reviewed-for: public-api

[JeanMeche]

I'm excited to see Angular's signal land as public API, thanks for the work on the developer Preview ❤️

[alxhub]

Caretaker: this will need a new g3 patch.

[jessicajaniuk]

reviewed-for: public-api

[alxhub]

This PR was merged into the repository by commit bc5ddab.

[goffxnca]

I miss the old days of the aesthetically pleasing and readable syntax of ref() in Vue and useState() in React for reactive synchronous values. It's inspiring to see how these three frameworks and libraries have influenced and inspired each other. I cannot wait to see these features publicly released! Kudos to you guys who are behind all of these amazing features.

[ivanivanyuk1993]

Added comment https://github.com/angular/angular/pull/49091/files#r1153925100

If we don't preprocess function in computed at compile-time or use some form of reflection to atomically notify only affected signals, won't it require O(n) change checks, where n is a quantity of signals in application, effectively making it have same algorithmic complexity as ChangeDetectionStrategy.Default and making it useless?

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