RPQVisitorParser()
Parse the RPQ ANTLR4 AST into our internal LogicalPlan model.
Auto-generated ANTLR files are stored in target/generated-sources/antlr4.
We use the Visitor approach instead of the Listener approach. Inspiration: http://jakubdziworski.github.io/java/2016/04/01/antlr_visitor_vs_listener.html