Return variables used within a PowerFx expression

[gekarapa]

As part of a static analysis feature, we need a service that can return all variables used within a PowerFx expression. This mainly supports the rule for detecting unused variables.
Example:
Given a valid PowerFx expression:
exactin(var1, var2)
The should return:
var1, var2

The input should be a valid PowerFx expression and should work for nested and complex expressions.

[anderson-joyle]

I plan to add this improvement on top of the dependency scan feature by adding a _variable_ key to the returned dict.
For example, given the following expression: Patch(simple2, First(simple1), { a : x })

Before: Entity Simple1: a, b; Entity Simple2: a, b;
After: "Entity Simple1: a, b; Entity Simple2: a, b; _variables_: simple2, simple1, x;"