[List start/reversed] Editing part

[Reinmar]

📝 Provide a description of the new feature

Part of #1032.

Follows #10540.

We changed the idea a bit during the refinement and will go for:

Idea: Instead of having a separate plugin, extend ListStyle and make it configurable which styles should be enabled: style, start, reversed

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[oleq]

Scope

• Note: The functionality of listReversed and listStart is very similar, we should optimize the code/helpers/logic to re-use things when possible.

listReversed attribute

• For the record: this attribute only concerns numbered lists.
• Modify ListStyleEditing
  • Extend the schema.
  • Upcast converter for the listReversed attribute.
  • Downcast converter for the listReversed attribute.
  • Add/extend the fixListStyleAttributeOnListItemElements() post-fixer for the listReversed attribute to make sure all list items have it.
  • Add/extend the _mergeListStyleAttributeWhileMergingLists() functionality to synchronize the listReversed attribute between merged lists.
  • Add/extend the removeListStyleAttributeFromTodoList post-fixer so the listReversed attribute does not leak to TODO lists.
  • Integration with indent/outdent: when indenting/outdenting the listReversed attribute should be cleaned/inherited from the parent list.
  • Restoring (clean-up) of the listReversed attribute when the type of the list changes.

listStart attribute

• For the record: this attribute only concerns numbered lists.
• Modify ListStyleEditing
  • Extend the schema.
  • Upcast converter for the listStart attribute.
  • Downcast converter for the listStart attribute.
  • Add/extend the fixListStyleAttributeOnListItemElements() post-fixer for the listStart attribute to make sure all list items have it.
  • Add/extend the _mergeListStyleAttributeWhileMergingLists() functionality to synchronize the listStart attribute between merged lists.
  • Add/extend the removeListStyleAttributeFromTodoList post-fixer so the listStart attribute does not leak to TODO lists.
  • Integration with indent/outdent: when indenting/outdenting the listStart attribute should be cleaned/inherited from the parent list.
  • Restoring (clean-up) of the listStart attribute when the type of the list changes.

Commands

• Modify ListStyleCommand
  • Add an argument to the constructor to let it know which model attribute it should operate on.
    • constructor( editor, attributeName, defaultValue )
  • There will be 3 instances of the command: one for listStyle, one for listReversed, and one for listStart
  • Let's make sure commands for reversed and start are disabled when an unordered list is selected
    • The UI will listen to this state.
    • Two possible implementations to choose from:
      • more options to the constructor
      • class inheritance with custom _checkEnabled()