Extend data schema item definition with allowChildren property

[jacekbogdanski]

📝 Provide a description of the improvement

Currently, to allow additional model children, we have to go with 2 steps:

1. Register new model schema definition:

editor.model.schema.register( 'custom-model', {
  allowIn: '$root'
} );

2. Extend existing model with allowIn property:

editor.model.schema.extend( '$text', {
  allowIn: 'custom-model'
} );

This code is so often used (especially for $text model) that we could improve it by extending current SchemaItemDefinition with additional member allowChildren which would simplify above code to:

editor.model.schema.register( 'custom-model', {
  allowIn: '$root',
  allowChildren: '$text'
} );

Of course, it can be used for other model elements, the feature is not restricted to $text node only:

editor.model.schema.register( 'custom-model', {
  allowIn: '$root',
  allowChildren: [ '$text', 'listItem' ]
} );

The new property should make sure if the children element is already registered or throw an error otherwise.

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