This also adds a variant of {add,remove}_event_listener called
{add,remove}_event_listener_with_options.
This is used internally to perform {add,remove}_event_listener with a
default constructed options struct. It was done like this because
default constructing the Variant with the options struct requires the
struct defintions to be present, which requires us to include
AbortSignal.h, which would cause a circular include as AbortSignal.h
includes EventTarget.h.
This is not a complete implementation of API, since we're also supposed
to accept an options dictionary as the third argument. However, a lot of
web content uses the boolean variant, and it's trivial to support.
