feat(signals): add `withLinkedState()`

[rainerhahnekamp]

This is a non-breaking feature to support linkedSignal.

This branch is based on #4795 which has to be merged first.

Please read the comment in #4871

---

withLinkedState generates and adds the properties of a linkedSignal to the store's state.

Usage Notes:

const UserStore = signalStore(
  withState({ options: [1, 2, 3] }),
  withLinkedState(({ options }) => ({ selectOption: options()[0] ?? undefined }))
);

The resulting state is of type { options: number[], selectOption: number | undefined }. Whenever the options signal changes, the selectOption will automatically update.

For advanced use cases, linkedSignal can be called within withLinkedState:

const UserStore = signalStore(
  withState({ id: 1 }),
  withLinkedState(({ id }) => linkedSignal({
    source: id,
    computation: () => ({ firstname: '', lastname: '' })
  }))
)

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• [x] 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 #4871

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[rainerhahnekamp]

@markostanimirovic, @timdeschryver this one has been rebased to main and is now ready for review.

[rainerhahnekamp]

Closing because of stalling github processing (more than 2 hours)

[rainerhahnekamp]

@markostanimirovic, @timdeschryver, I think I've applied all necessary changes (including the docs). We can do another round of review.

[rainerhahnekamp]

@markostanimirovic, @michael-small

Wow, what a review. Corrections outnumbered the actual code by a margin...a review for the records 😅 . I seriously owe you both a drink for that.

---

I've applied all the changes and double-checked. I also updated the jsdoc for withLinkedState so that it matches the examples of the docs.