Add optional fix for unreachable medpack in Xian

[Richard-L]

There's the infamous unreachable large medpack in Xian which has caused people to come up with setups to get there: https://youtu.be/ZReAfsNYt40

It's because the ladder leading up there isn't climbable. Most people believe this is an oversight in the level data. @lahm86 would there be anything in your eyes that confirms this?

TRR has changed the ladder to be climbable, and I'd suggest it may be a desirable option for TR2X too.

[lahm86]

I don't think there is a way to tell from the data if it was just an oversight; the one in Midas in TR1 is questionable too, then the fact there is another unreachable one in TR3 makes you think it was a running joke. But I have heard too that it was an oversight here. In any case, it's easy to fix.