feat(signals): add return type to the `set` operation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.65%. Comparing base (83a01b4) to head (73d87af). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/ts/react-signals/src/ValueSignal.ts	83.33%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2889 +/- ## ========================================== - Coverage 92.68% 92.65% -0.04% ========================================== Files 83 83 Lines 2830 2831 +1 Branches 728 729 +1 ========================================== Hits 2623 2623 Misses 156 156 - Partials 51 52 +1 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2889/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/vaadin/hilla/pull/2889/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `92.65% <83.33%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin#carryforward-flags-in-the-pull-request-comment) to find out more.

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[sonarcloud[bot]]

Quality Gate failed

Failed conditions
33.3% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud