fix: correctly add values to self.__grid instead of the grid class table

lib/level/grid/base.lua

@@ -8,7 +8,7 @@ grid.__index = grid
 --- adds a value to the grid
 --- @param value any
 function grid:add(value)
-    grid[tostring(value.position)] = value
+    self.__grid[tostring(value.position)] = value
 end
 
 --- @param position Vec3