Add getter api to @xstate/store

[expelledboy]

Merge getters implementation from #5183 onto #5175

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[expelledboy]

Obv you get a circular reference between the getters type and the second arg of its definition.

Depending on how many second pass getters you have there are two approaches

Define the type of ctx for parents

const store = createStore({
  context: { price: 10, quantity: 2 },
  getters: {
    subtotal: (ctx: { price: number; quantity: number }) =>
      ctx.price * ctx.quantity,
    tax: (_, getters) => getters.subtotal * 0.1,
    total: (_, getters) => getters.subtotal + getters.tax
  },
  on: {
    updatePrice: (ctx, ev: { value: number }) => ({ price: ev.value })
  }
});

Or just define the type of getters

const store = createStore({
  context: { price: 10, quantity: 2 },
  getters: {
    subtotal: (ctx) => ctx.price * ctx.quantity,
    tax: (_, getters: { subtotal: number }): number =>
      getters.subtotal * 0.1,
    total: (_, getters: { subtotal: number; tax: number }): number =>
      getters.subtotal + getters.tax
  },
  on: {
    updatePrice: (ctx, ev: { value: number }) => ({ price: ev.value })
  }
});

Probably would want to include this in the docs.

Unless anyone has a better idea on the API here.

[expelledboy]

@davidkpiano Please can you assist me merging this? I dont want there to be massive merge conflicts in 2-4 months with the v3 branch.

Mostly I need assistance getting the types working in the surrounding utils.

And / or if you agree to the API itself.