* refactor(events): Introduce type predicates for event classes
Introduce predicates for testing Abstract event subclasses based on
their .type properties. These are useful because there are places
where it is not possible to use instanceof <ClassConstructor> tests
for type narrowing due to load ordering issues that would be caused
by the need to import (rather than just import type) the class
constructors in question.
* refactor(events): Use event type predicates
Simplify several sections of code by using type predicates for
type narrowing and thereby avoiding the need for explicit casts.
* chore(events): Fix copyright date of recently-added files
* chore: Remove unused import
032b5ed9ea