Command Parser

[cchannon]

We need to implement a new Parser function in the plugin that--using engine.Parse--can inspect an arbitrarily complex PowerFx command and handle specific command inputs that should be supported but otherwise would cause errors.

The Parser should recursively dig through Functions and Args until it reaches any Lookup declaration. That Lookup declaration would then need to be evaluated and converted into a Query Expression so we can--on the fly-- build an in-memory list of the top 5k results from any table in Dataverse and make those records accessible to the PFX author. This will also mean updating the CompareContext method, and an additional Issue will need to be created to handle that.