1640: Remove conversion.register() add conversion.addAlias()

[jodator]

Suggested merge commit message (convention)

Other: Change Conversion class API. Closes ckeditor/ckeditor5#4467.

BREAKING CHANGE: The Conversion#register() method was removed from public API. Use constructor parameters to pass dispatchers and Conversion#addAlias() to register an alternative conversion group for registered upcast or downcast dispatcher.

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Additional information

• As agreed in https://github.com/ckeditor/ckeditor5-engine/issues/1640#issuecomment-461078858 I've changed the conversion API to:

   this.conversion = new Conversion(
   	[ this.editing.downcastDispatcher, this.data.downcastDispatcher ],
   	this.data.upcastDispatcher
   );
   this.conversion.addAlias( 'dataDowncast', this.data.downcastDispatcher );
   this.conversion.addAlias( 'editingDowncast', this.editing.downcastDispatcher );

• constellation of change on ckeditor5#t/ckeditor5-engine/1640 branch and the CI is green

• Related PR In the core: Align code to the changes in Conversion API. ckeditor5-core#162.

[coveralls]

Coverage decreased (-8.4%) to 91.578% when pulling c02a29f on 1640 into c31bea6 on master.

[pjasiun]

It needs to be simplified :)

[pjasiun]

Is it efficient to create a new instance whenever the converter is used? Maybe we could store instances of helpers?

[jodator]

Since we only have upcast and downcast types of dispatchers - which have to be present anyway - I've changed the way how the conversion helpers and dispatchers are stored.

So the both groups are stored in separate private fields (easier & faster to find if dispatcher was registered in one of group - simpler code).

The dispatcher groups/aliases are stored in _helpers map as they are returned from conversion.for() method.