[core] Add schema

[davidkpiano]

This PR adds the schema property to the machine config and definition.

const machine = createMachine({
  // ...
  schema: {
    // These can be arbitrary meta data, accessible at runtime
    // Typecast these (something as SpecificType) for inferred types!
    context: someContextSchema,
    events: someEventSchemaMap,
    actions: someActionSchemaMap,
    guards: someGuardSchemaMap,
    services: someServiceSchemaMap
  },
  // ...
});

Purpose of this PR:

• Provides a place to specify important metadata for runtime types and validation (e.g., with JSON Schema) as part of the machine definition
• Allows for easier type inference without having to specify generics. Notice how the context and events are strongly typed, even though their generic types aren't specified:

[changeset-bot]

🦋 Changeset detected

Latest commit: 907d577

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

This PR includes changesets to release 3 packages

Name	Type
xstate	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR