I/6152: Added 'mouseup', 'mousemove' and 'mouseleave' events to MouseObserver.

[jodator]

Suggested merge commit message (convention)

Enhancement: Added 'mouseup', 'mousemove' and 'mouseleave' events to MouseObserver. Closes ckeditor/ckeditor5#6152.

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Additional information

• This PR adds events required by table selection feature. At this stage, it is not required by its own.
• The events in table selection are used to:
  • 'mouseup' - stop selecting table cells
  • 'mousemove' -
  • 'mouseleave'

[coveralls]

Coverage remained the same at 100.0% when pulling 7f4d92a on i/6152 into 48bea53 on master.

[jodator]

Actually it makes no sense to make this now in the engine as we can introduce this in table selection.