Add watched and unwatched

[JoviDeCroock]

Resolves #351
Resolves #428

This adds a second argument to signal which are the options, currently this adds two properties, watched and unwatched. These will most likely serve third party libraries a lot more than first party consumers.

• watched will trigger when the first subscriber subscribes to a signal
• unwatched will trigger when the last subscriber unsubscribes from a signal

The second parameter can in the future be used for other RFC's in our pipeline like the equality function. The addition of this parameter is in line with the TC39 proposal for browser-native signals https://github.com/tc39/proposal-signals

I opted not to add this to computed for the time being as the signal itself is the atom while computed is more of a derived value. Not sure if a utility like this would be useful there. In the TC39 proposal they also add this to computed, so might as well for parity.

Note that calling the watcher on the subscribe is slightly distinct from how we normally treat signals, i.e. we calculate the computed when its value is accessed for the first time.

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🦋 Changeset detected

Latest commit: ee34c3c

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

This PR includes changesets to release 3 packages

Name	Type
@preact/signals-core	Minor
@preact/signals	Minor
@preact/signals-react	Minor

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Size Change: +197 B (+0.24%)

Total Size: 83.5 kB

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