feat(finance): add economic module with public/private debt modeling

[Zhanwei-Liu]

This commit introduces a new finance module that supports technology-
and zone-specific capital structure modeling, including:

• New input parameters: public/private debt ratio, cost of public/private
  debt, cost of private equity
• Calculation of zone- and technology-specific weighted average cost of
  capital (WACC)
• Constraints for public debt investment (zone-level and system-wide)
• Output of technology- and zone-specific public debt capital expenditure

Users can configure these parameters in "public_debt_ratio.xlsx",
"private_debt_ratio.xlsx", "cost_of_public_debt.xlsx", etc., and view
results of public debt investment per year and per zone.