Include easyNPC documentation

[vilarion]

easyNPC is a simple language to write NPCs for Illarion. It is currently documented in the easyNPC editor, which is subject to removal as per Illarion-eV/Illarion-Java#127.

That documentation should probably be included here, since those NPC scripts are translated into Lua and thus closely related to Illarion scripting. It should be mentioned and linked to in the introduction and receive a top level chapter between "Classes" and "Common Bugs", I think.

[estralis]

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