[CVaR]: Add slack variable that captures tail excess losses

[datejada]

Description

• Slack variable for s ∈ S that captures tail excess losses ($\xi$). Use descriptive name tail_excess_slack_xi
  • Add a new file for this variable here (reuse the same file as in [CVaR]: Add VaR threshold variable #1525) https://github.com/TulipaEnergy/TulipaEnergyModel.jl/tree/main/src/variables
  • Add the corresponding SQL query: https://github.com/TulipaEnergy/TulipaEnergyModel.jl/blob/main/src/sql/create-variables.sql
  • Only create the new variable if the value of the $\lambda$ parameter (see [CVaR]: Add new model parameter for risk aversion #1524) is greater than zero
• The variable is continuous and positive (double check for upper bound)
• It depends on the scenario index, and the units are the currency of the objective function
• Add a test that checks that the variable is created when the $\lambda$ parameter is greater than zero