Add variable resistor component

[langestefan]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add a variable resistor component with optional temperature dependency using a HeatPort.

Based on the potentiometer implementation from Modelica: https://doc.modelica.org/Modelica%203.2.3/Resources/helpWSM/Modelica/Modelica.Electrical.Analog.Basic.Potentiometer.html

However I named the component VariableResistor as opposed to potentiometer because the component has two ports, not three.

[matthew-kapp]

@langestefan @BenChung @ChrisRackauckas I keep getting an error from this component during tests. Have you come across this as well?

Testing Running tests... Unbound type parameters detected: [1] var"#__VariableResistor__#18"(name, T_dep, enforce_bounds, R_ref::Union{Missing, Nothing, ModelingToolkit.NoValue, var"##608"}, T_ref::Union{Missing, Nothing, ModelingToolkit.NoValue, var"##609"}, R_const::Union{Missing, Nothing, ModelingToolkit.NoValue, var"##610"}, pos::Union{Missing, Nothing, ModelingToolkit.NoValue, Real}, R::Union{Missing, Nothing, ModelingToolkit.NoValue, var"##611"}, alpha::Union{Missing, Nothing, ModelingToolkit.NoValue, var"##612"}, ::typeof(ModelingToolkitStandardLibrary.Electrical.__VariableResistor__)) where {var"##608", var"##609", var"##610", var"##611", var"##612"} @ ModelingToolkitStandardLibrary.Electrical none:0 Aqua: Test Failed at C:\Users\matth\.julia\packages\Aqua\epbUr\src\unbound_args.jl:37