Add initial implementation of thorns

[madxmike]

Fixes #760 .

Description of the problem being solved:

This PR adds an initial implementation of the thorns mechanic. See #760

We detect if any thorns damage adding mods are active on the player's gear. If so, we grant a custom "ThornsPlayer" skill. This is inspired with how explode corpses is implemented in pob1.

DRAFT: Putting this up now as I have the skill and mods working, but Im not sure if this is the best way to implement this. Wanting feedback. I still need to test this mechanic a lot in game to get a full grasp of the mods that do and do not effect thorns. So ailments and other on-hit type things will certainly need to be adjusted before this is final.

Steps taken to verify a working solution:

• Loaded build below and viewed thorns damage calcs

Link to a build that showcases this PR:

https://maxroll.gg/poe2/pob/ln53d00j

Before screenshot:

After screenshot:

[Nostrademous]

I started it, but never finished. I am assuming you are tracking #610