Add code linting with luacheck (#221)

With luacheck mistakes in Lua code can be found, e.g. the use of undefined variables, and the code style can be checked.
workflow by @Panquesito7

.luacheckrc

@@ -0,0 +1,8 @@
+read_globals = {"minetest", "vector", "VoxelArea", "ItemStack",
+	"table",
+	"unified_inventory", "sfinv", "smart_inventory", "inventory_plus"
+}
+globals = {"worldedit"}
+-- Ignore these errors until someone decides to fix them
+ignore = {"113", "211", "212", "213", "311", "312", "411", "412", "421", "422",
+	"431", "432", "611", "614", "621", "631"}