wielded_light mod for Minetest

Idea taken from torches_wieldlight in https://github.com/minetest-mods/torches, but written from scratch and usable for all shining items.

All bright nodes with light value > 3 lighten the player environment if wielded, with value fewer by 3. (Torch 13->10 for example)

Dependencies: none

License: GPL-3