Add a rule `no-comparison-of-unary-test`

[barmac]

Is your feature request related to a problem? Please describe.

This is a valid FEEL expression: foo == bar
However, it will always evaluate to null. That's because we are comparing variable foo to an unary test: = bar.
This will bite any FEEL novice who wrote code before as most programming languages use double or even triple = for comparisons.

Describe the solution you'd like

Warn a user when they pass a unary test to a comparison:

Comparison [0, 10]
  VariableName [0, 3]
  CompareOp [4, 5]
  SimplePositiveUnaryTest [5, 10]
    CompareOp [5, 6]
    VariableName [7, 10]

Describe alternatives you've considered

Require to enclose unary test in parenthesis. However, that does not solve the problem, and is cumbersome when we actually want to use an inline unary test: foo in (100, >100)

Additional context

Discovered in bpmn-io/properties-panel#394