Add source code and include lua_async as submodule

.gitmodules

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+[submodule "src"]
+	path = src
+	url = https://github.com/EliasFleckenstein03/lua_async

init.lua

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+local path = minetest.get_modpath(minetest.get_current_modname()) .. "/src/"
+
+dofile(path .. "init.lua")(path)
+
+minetest.register_globalstep(function(dtime)
+	lua_async.step(dtime)
+end)
+
+function lua_async.mt_validate_objs(...)
+	for _, ref in ipairs {...} do
+		if not ref:is_player() and not ref:get_luaentity() then
+			coroutine.yield(true)
+		end
+	end
+end

mod.conf

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+name = lua_async

src

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+Subproject commit a2770298f040307f8dd59c7a88d7e40d37faec14