feat(signals): rename signals to computed when defining custom features with input

[markostanimirovic]

PR Checklist

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)
• [x] 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?

To define computed input, the signals property name is used:

function withN() {
  return signalStoreFeature(
    { signals: type<{ l: Signal<number>; m: Signal<number> }>() },
    withComputed(({ l, m }) => ({
      n: computed(() => l() + m()),
    }))
  );
}

Closes #4391

What is the new behavior?

To define computed input, the computed property name is used:

function withN() {
  return signalStoreFeature(
    { computed: type<{ l: Signal<number>; m: Signal<number> }>() },
    withComputed(({ l, m }) => ({
      n: computed(() => l() + m()),
    }))
  );
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This is not considered as a breaking change because the @ngrx/signals package is in developer preview.

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