feat(signals): add signalState and selectSignal APIs

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #3993

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[brandonroberts]

Does this type need to be exported?

[markostanimirovic]

Yes, it can be used to avoid manual typing when creating custom updaters. For example:

import { SignalStateUpdater } from '@ngrx/signals';

export function addOne<T>(entity: T): SignalStateUpdater<{ entities: T[] }> {
  return (state) => ({ entities: [...state.entities, entity] });
}

// usage:
import { signalState } from '@ngrx/signals';

type State = { entities: User[] };

const usersState = signalState<State>({ entities: [] });

usersState.$update(
  addOne({ name: 'Marko' }),
  addOne({ name: 'Brandon' })
);