Provides code-driven workflow functionality.

A workflow is a set of states and transitions that an entity goes through during its lifecycle.
A transition represents a one-way link between two states and has its own label.
The current state of a workflow is stored in a state field, which provides an API for getting and
applying transitions. An entity can have multiple workflows, each in its own state field.
An order might have checkout and payment workflows. A node might have legal and marketing workflows.
Workflow groups are used to group workflows used for the same purpose (e.g. payment workflows).

## Architecture
[Workflow](https://github.com/bojanz/state_machine/blob/8.x-1.x/src/Plugin/Workflow/WorkflowInterface.php) and [WorkflowGroup](https://github.com/bojanz/state_machine/blob/8.x-1.x/src/Plugin/WorkflowGroup/WorkflowGroupInterface.php) are plugins defined in YAML, similar to menu links.
This leaves room for a future entity-based UI.

Example yourmodule.workflow_groups.yml:
```yaml
commerce_order:
  label: Order
  entity_type: commerce_order
```
Groups can also override the default workflow class, for more advanced use cases.

Example yourmodule.workflows.yml:
```yaml
default:
  id: default
  label: Default
  group: commerce_order
  states:
    new:
      label: New
    fulfillment:
      label: Fulfilment
    completed:
      label: Completed
    canceled:
      label: Canceled
  transitions:
    create:
      label: Create
      from: [new]
      to:   fulfillment
    fulfill:
      label: Fulfill
      from: [fulfillment]
      to: completed
    cancel:
      label: Cancel
      from: [new, fulfillment]
      to:   canceled
```

Transitions can be further restricted by [guards](https://github.com/bojanz/state_machine/blob/8.x-1.x/src/Guard/GuardInterface.php), which are implemented as tagged services:
```yaml
  mymodule.fulfillment_guard:
    class: Drupal\mymodule\Guard\FulfillmentGuard
    tags:
      - { name: state_machine.guard, group: commerce_order }
```
The group argument allows the guard factory to only instantiate the guards relevant
to a specific workflow group.

The current state is stored in a [StateItem](https://github.com/bojanz/state_machine/blob/8.x-1.x/src/Plugin/Field/FieldType/StateItem.php) field.
A field setting specifies the used workflow, or a value callback that allows
the workflow to be resolved at runtime (checkout workflow based on the used plugin, etc.
A validator is provided that ensures that the specified state is valid (exists in the
workflow and is in the allowed transitions).

The current state should always be changed by applying a transition:
```php
// Apply the "complete" transition to field_state.
$state_item = $entity->get('field_state')->first();
$state_item->applyTransitionById('complete');
$entity->save();

// Or, if the entity has a getter...
$entity->getState()->applyTransitionById('complete');
$entity->save();
```
This allows the next state ID to vary between different workflows (as long as they all have the same transition).

A formatter is provided that outputs a form with the allowed transitions,
allowing workflow changes to happen outside of the edit form.

## Events
If a transition has been applied, the StateItem field will dispatch several events on entity save.
The pre_transition events are dispatched before the save (and allow the entity to be modified), while
the post_transition events are dispatched after the save.

### Transition-specific

Pattern: "{$workflow_group}.{$transition}.{$phase}"

Examples:
- commerce_order.create.pre_transition
- commerce_order.create.post_transition

### Group-specific

Pattern: "{$workflow_group}.{$phase}"

Examples:
- commerce_order.pre_transition
- commerce_order.post_transition

Useful for performing an action based on the "to" state, regardless of which
transition made the change.

### Generic

Pattern: "state_machine.{$phase}"

Examples:
- state_machine.pre_transition
- state_machine.post_transition

Useful for logging, notifications, and other use cases that require reacting
to every transition regardless of the workflow group.

Credits
-------
Initial code by Pedro Cambra.

Inspired by https://github.com/winzou/state-machine