If no value argument is given, /waterquality should just return the current value

Currently, the `/waterquality` command can be used to set the water quality of a specific water source using the `/waterquality <tag> <0-100%>` syntax. It would be helpful to also have a way to query the current water quality without changing it. The simplest way to add this would probably be to make it so that `/waterquality <tag>` returns a message like "The water quality of <tag> is <0-100%>". This would be helpful both for server bots and for server owners testing out different water quality config settings.