Allow setting a signal value inside a computed

[jviide]

This pull request allows setting a signal's value inside a computed. Previously the following code was not allowed and raised an error with the message "Computed cannot have side-effects":

const s = signal(0);

const c = computed(() => {
  s.value = 1;
});

c.value; // Error: Computed cannot have side-effects

This suggested change is based on Slack discussions (TL;DR: there may be legitimate use-cases that the side-effect limitation blocks / the "regular" cycle detection might be enough for preventing runaway modification loops) and submitted here for further evaluation. /cc @developit @marvinhagemeister

This change saves some bytes from the output bundles:

Before this change:
       1481 B: signals-core.js.gz
       1350 B: signals-core.js.br
       1494 B: signals-core.mjs.gz
       1363 B: signals-core.mjs.br
       1488 B: signals-core.module.js.gz
       1352 B: signals-core.module.js.br
       1557 B: signals-core.min.js.gz
       1414 B: signals-core.min.js.br

After this change:
       1439 B: signals-core.js.gz
       1312 B: signals-core.js.br
       1452 B: signals-core.mjs.gz
       1325 B: signals-core.mjs.br
       1448 B: signals-core.module.js.gz
       1315 B: signals-core.module.js.br
       1518 B: signals-core.min.js.gz
       1381 B: signals-core.min.js.br

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🦋 Changeset detected

Latest commit: cb6bdab

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@preact/signals-core	Minor

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