Missing "VolumeOfLiquid" var

[kaedys]

Most pipe analyzers, and most machines that provide values on their inputs/outputs (ex. Air Conditioning, Filtration, etc), provide a VolumeOfLiquid var as an available reading. This var is not currently supported by the plugin (and probably also the language server, so may require an upstream change)

(Note: the additional error highlight at the end of the line is unrelated, and due to #8 instead)