[core] Add meta types

[davidkpiano]

Meta objects for state nodes and transitions can now be specified in setup({ types: … }):

const machine = setup({
  types: {
    meta: {} as {
      layout: string;
    }
  }
}).createMachine({
  initial: 'home',
  states: {
    home: {
      meta: {
        layout: 'full' // strongly typed
      }
    }
  }
});

const actor = createActor(machine).start();

actor.getSnapshot().getMeta().home!;
// typed as { layout: string }

Fixes #809

[changeset-bot]

🦋 Changeset detected

Latest commit: f546669

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

This PR includes changesets to release 1 package

Name	Type
xstate	Minor

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[jonestristand]

This would be very useful to me!

[davidkpiano]

cc. @Andarist Quick review? 🥺

[jonestristand]

Amazing, thank you!