[2.x] allow to expect toThrow against an exception instance

[fabio-ivona]

This PR implements Throwable support as argument of a toThrow expectation. This will allow to:

expect(fn() => $user->subscribe($team))
        ->toThrow(new TeamException("user is already subscribed to team"));

in this case, the expectation will pass if the Throwable is an instance of TeamException and has user is already subscribed to team as exception message

this will allow a fast check when using static exception constructors like the following one, without the need to enter the code and check the exception message:

expect(fn() => $user->subscribe($team))
        ->toThrow(TeamException::alreadySubscribed());

[nunomaduro]

Good job here. Can you add docs?