[@xstate/store] Store select

[davidkpiano]

Added selectors to @xstate/store that enable efficient state selection and subscription:

• store.select(selector) function to create a "selector" entity where you can:
  • Get current value with .get()
  • Subscribe to changes with .subscribe(callback)
  • Only notify subscribers when selected value actually changes
  • Support custom equality functions for fine-grained control over updates via store.select(selector, equalityFn)

const store = createStore({
  context: {
    position: { x: 0, y: 0 },
    user: { name: 'John', age: 30 }
  },
  on: {
    positionUpdated: (
      context,
      event: { position: { x: number; y: number } }
    ) => ({
      ...context,
      position: event.position
    })
  }
});

const position = store.select((state) => state.context.position);

position.get(); // { x: 0, y: 0 }

position.subscribe((position) => {
  console.log(position);
});

store.trigger.positionUpdated({ x: 100, y: 200 });
// Logs: { x: 100, y: 200 }

[changeset-bot]

🦋 Changeset detected

Latest commit: 0979f8d

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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[davidkpiano]

@Andarist Final review? 🥺