breaking: remove $state.link callback

[Rich-Harris]

See #12938 (comment) (and #12545 for the original PR).

I regretted this API almost as soon as I merged it. Short of removing $state.link altogether, I'm pretty certain that we should remove the callback version — if you need to do something more complicated than having a local piece of state immediately update when some other state changes, use an effect. Whether or not a dedicated API for the remaining use case (which is very narrow) is warranted is a live question, I think, but hopefully we can get agreement on this more limited change before too much damage is done from having the current version in the wild.

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[trueadm]

Agreed 100%

[Rich-Harris]

See also #12943

[Rich-Harris]

Releasing this pending resolution on the broader question of #12943