major: Switch to forward slashes(+more) to fix compilation on Linux (#1403)

Notably also adds some metadata files for NixOS 

* add support for linux clang cross compiles

* add linux clang instructions

* un-capitalize Mob.horse.*

* update the description in flake.nix

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Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/LightningBoltRenderer.cpp

@@ -1,7 +1,7 @@
 #include "stdafx.h"
 #include "LightningBoltRenderer.h"
 #include "Tesselator.h"
-#include "..\Minecraft.World\net.minecraft.world.entity.global.h"
+#include "../Minecraft.World/net.minecraft.world.entity.global.h"
 
 void LightningBoltRenderer::render(shared_ptr<Entity> _bolt, double x, double y, double z, float rot, float a)
 {