feat: add insert variables dropdown for visualizations

Dennis Collarisrequested to merge
feat/lexical-insert-variables-0dvis into main

Referencing asana task https://app.asana.com/0/1208418775411311/1208446743427522

This PR enables inserting a variable node into the editor representing visualizations. I introduced a VariableType enum to distinguish between stats and visualizations (otherwise ambiguity emerges when a vis has the same name as a stat).

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