ref: Expose the ExceptionMechanism through the event hint

[cleptric]

This makes it possible to access the handled property when reporting errors to Sentry.
While this approach is definitely not the best solution, I didn't find a better way without breaking BC.

$hint = EventHint::fromArray([
    'exception' => $exception,
    'mechanism' => new ExceptionMechanism(ExceptionMechanism::TYPE_GENERIC, false),
]);

captureEvent(Event::createEvent(), $hint);

Remaining ToDos

• Add more tests

Closes #1350