I was sad to find Julia doesn't have a TreeSHAP implementation so I ported FastTreeSHAPv1 for my favorite boosted trees library. This PR is in case others find it useful.

This is marked as a draft for a few reasons:

1. To gauge interest to see if SHAP values should be in this package at all
2. The code runs but the results haven't yet been tested or compared to other SHAP value libraries yet
3. I'm not very familiar with the FastTreeSHAP algorithm, this was a simple 1:1 port of the pseudocode. So suggestions are very welcome
4. There may be a better way to parallelize this without using Channels. I usually use a tmap implementation for these kinds of parallel tasks, but I didn't want to add dependencies to the package just for this. Again, suggestions are welcome.