Add support for atomic transactions

[PsychoLlama]

This is an implementation for my feature request in issue 460. It adds a new function called transaction(...) which behaves similar to batch(...), except it rolls back all the state changes if an error is thrown.

const a = signal('original')

try {
  transaction(() => {
    a.value = 'changed'
    throw new Error('kaboom')
  })
} catch {
  a.value // 'original'
}

Intermediate states aren't observable until the transaction is complete.

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I know this change is public facing so I'm not expecting it to be straightforward. Think of the PR as a proof of concept and feel free to say no.

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[PsychoLlama]

Closing - see related issue for discussion.