fixes#5858
technically speaking, the sideways states for non-fully-grown stems shouldn't exist, but they do in Bedrock, and changing this code to split non-fully-grown stems from fully grown ones would likely require BC breaks.
This was the minimum necessary to achieve the desired functionality.
due to technical limitations, this requires separating them back into two different block types. However, this isn't too egregious since it's just one flag, and actually simplifies some code.
closes#5478
this doesn't match the 1.16+ behaviour yet, but it at least recognizes walls that are already in the post-1.16 way and doesn't break them if not interacted with.
