(1.21.1) Vertical slab model orientation

[throckmorpheus]

Previously, we were generating all the vertical slab models by rotating the bottom slab model, which made certain vertical double slabs look strange. I've switched it to using the top slab model for the positive half and the bottom slab model for the negative half.

Before:

After:

This rotation method still leaves us with weird-looking differences between how vertical slabs behave as opposed to normal slabs & stairs, so we should probably switch to attempting to load a dedicated vertical slab model and then falling back to the rotated horizontal slab model (so one could fix that with a resource pack), but I don't know how to do that so I'm leaving it here for now.

[CJMinecraft01]

The slabs look soo much better with these changes but I'm a bit unsure on the approach here as it relies on the fact that the top variant of the slab has the _top suffix in the resource name which cannot always be guaranteed. Also, swapping from using the BlockState as the key to using the Block means that if a modded slab uses a variant property on the state (e.g., an enum for the direction of the slab), we only use the default variant. I'll have a look at doing this and I'll put up a PR - thanks for raising this!!

Also, could you expand on what you mean by "weird-looking differences between how vertical slabs behave as opposed to normal slabs & stairs"? Could you show an example?

[CJMinecraft01]

I've had a shot at doing this by using the correct model for each half - see #326