Add GetItemTypePrice and a forward for GetItemTypeMaxHitPoints

[mysy00]

There was a forward missing for GetItemTypeMaxHitPoints function. Hence, I hadn't had added this function.
In this pull request I add the missing forward + GetItemTypePrice.

I'd also like to ask. Is there any particular reason why ItemTypes lack of setters? I was considering adding them in the future.
I feel like it would be nice to dynamically manage not only items but also whole item types.

[Southclaws]

Thanks!

And the reason there aren't many setters for item type data is that the original design behind item type data is that it remains static. This allows you to build against a set of assumptions and it simplifies everything. There also was never any user cases for introducing the ability to change some property about every single instance of an item. For example, you probably wouldn't ever need to change the model, weight, object offsets or hitpoints for every single AK-47 in the entire server. And if you do, it's always a long-term change not something that's only going to take affect for a few minutes so it warrants a re-compile of the game.

If there are use-cases, it could be implemented but those changes would need to be carefully propagated across all instances and it may cause a lot of additional complexity to be required.