[ENHANCEMENT] Accept `${_}` variable as purposefully not used

[gohierf]

Is your enhancement request related to a problem? Please describe. I get the following warning: Variable '${_}' is not used.robotcode.diagnostics(VariableNotUsed)

For instance, when ignoring some of the output of a multi-output keyword.

Robocop unused-variable rule supports this convention.

Describe the solution you'd like I would like Robotcode to ignore ${_} when checking for unused variables.

Describe alternatives you've considered I could use # robotcode: ignore.

Additional context I think the convention of using _ for throwaway variables in Python is worth adding to Robotcode.

[d-biehl]

That's a good point!!!