Add configuration to show/hide metavar labelling

[jacobobq]

The metavar labelling is very useful when you are writing rules but in many contexts it adds a lot of visual noise. It be amazing to have a way to configure it from the extension settings.

[ajbt200128]

We're planning on launching a generally available version of the extension, but one of the features that got cut was metavar labelling, but we're planning on adding this back (and some other dataflow analysis related goodies) in the future!