[RFC][DX][Workflow] Configuration of initial places and marking store

@pbowyer

Description

In [Workflow] Deprecate MultipleStateMarkingStore and SingleStateMarkingStore in favor of MethodMarkingStore #30551, the marking store has been "simplified" internally, but the configuration is now more confusing (ref [Workflow] Deprecate MultipleStateMarkingStore and SingleStateMarkingStore in favor of MethodMarkingStore #30551 (review)).
In [Workflow] Added support for many inital places #30468, initial_place has been renamed initial_places but still supports passing one place that will be normalized to an array despite this single example in the upgrade file. Also, this naming is now a bit confusing too, whether we can use it with one place is not obvious anymore.

Proposition

Simplify the method type configuration which is the only one built-in we want to support now, and infer the single state from the type of the workflow, this leaves one argument to configure, the property name:
```
framework:
 workflows:
     article:
         type: state_machine # or workflow for multiple states
         marking_store:
             type: method # this line can be removed in Symfony 5.0
             property: state
```
Instead of inferring we can use another key: state (single or multiple) along with property,
or use a boolean as suggested by @pbowyer in the review linked above.
We could rename initial_places to something invariant like initial_marking or initial_state.

Any other suggestions? What do you think? ping @javiereguiluz @lyrixx

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