* Allow for priority to be set on event emitters
* Add payment event emitters
* add new actions/status to checkout state context
* implement event emitters in payment data context
* refactor checkout state context to use new actions in event emitters
* refactor checkout processor to handle new event emitters
* fix type-defs for registered payment methods
* register observer for cheque payment method integration
* add inline todo
* fix sort
* fix tests and add test for priority usage in event emitters
* remove todo and just add explanatory comment
* condense sort logic
* lowercase test description
* abstract emitter callback to reduce code
* don’t process passed in errors if it’s undefined
* improve error response expectation for payment processing event observer
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