[Lua] Expose container capacity and current volume

It'd be nice to be able to get a container's current volume (in L) as well as it's max volume (capacity) through lua. The current volume can be calculated if there's only 1 type of item (or all types have the same density) in the container and you know the density but it would be nice/better if we could just call getCapacity() and getCurrentVolume() (or something similar) rather than having to hardcode the max volume every time and calculate the current capacity as/when needed.

I don't imagine this would be super complex to add as it's exposed in the container contents UI already.

Done Suggested by: Thorrak Upvoted: 28 Feb, '21