[8.x] Add default parameter to throw_if / throw_unless (attempt 2)

[SjorsO]

This is a follow-up on #35888

You can now pass throw_if and throw_unless a message as the second parameter, similar to the report function (which was changed in #35803):

// Currently:
throw_if($sometingIsWrong, new RuntimeException('something wrong with user '.$user->id));

// With this PR:
throw_if($sometingIsWrong, 'something wrong with user '.$user->id);

If the message passed is an existing class, that will be used as the exception instead:

throw_if($somethingIsWrong, LogicException::class);

And the newly added default parameter allows you to also do this, if you're not interested in a custom message:

throw_if($somethingIsWrong);