[xstate/store] Atoms

[davidkpiano]

Added createAtom() for creating reactive atoms that can be combined with other atoms and stores:

• Create simple atoms with initial values:

  import { createAtom } from '@xstate/store';
  
  const countAtom = createAtom(0);
  countAtom.get(); // 0
  countAtom.set(1); // or use setter function: (prev) => prev + 1

• Subscribe to atom changes:

  countAtom.subscribe((value) => console.log(value));

• Combine multiple atoms:

  const nameAtom = createAtom('hello');
  const countAtom = createAtom(3);
  const combinedAtom = createAtom((read) =>
    read(nameAtom).repeat(read(countAtom))
  );
  combinedAtom.get(); // "hellohellohello"

• Seamlessly combine atoms with stores:

  const countAtom = createAtom(0);
  const nameStore = createStore({
    context: { name: 'David' }
    // ... store config
  });
  
  const combinedAtom = createAtom(
    (read) => read(nameStore).context.name + ` ${read(countAtom)}`
  );
  combinedAtom.get(); // "David 0"

Atoms automatically update when their dependencies change, making it easy to create derived state from both atoms and stores.

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🦋 Changeset detected

Latest commit: 4c518d3

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

This PR includes changesets to release 1 package

Name	Type
@xstate/store	Minor

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