Expose annual employee CPP/QPP/EI/QPIP contribution variables

[PavelMakarchuk]

Problem

PolicyEngine Canada does not expose standalone annual employee-liability
variables for:

• Canada Pension Plan contributions
• Quebec Pension Plan contributions
• Employment Insurance premiums
• Quebec Parental Insurance Plan premiums

These outputs are needed both by downstream disposable-income calculations and
by source-level comparison against CRA payroll formulas. Related work exists in
#152 (CPP), #403 (EI), and #527 (QPP), but QPIP and the common output contract
are not covered.

Scope

• Add person-level annual employee contribution/premium variables for CPP, QPP,
  EI, and QPIP.
• Apply province-residence routing between CPP/QPP and EI/QPIP.
• Parameterize the basic exemption, YMPE, YAMPE, rates, and annual maxima by
  effective year.
• Represent base and additional CPP/QPP contributions explicitly where needed,
  while also exposing total employee liability.

Acceptance criteria

• Each program has a stable person-level annual output variable.
• Tests cover zero earnings, the basic exemption, YMPE, YAMPE, and annual caps.
• Quebec and non-Quebec cases exercise the correct mutually exclusive pension
  plan and applicable insurance premiums.
• Parameter metadata cites the current CRA payroll formulas.

Official source:
https://www.canada.ca/en/revenue-agency/services/forms-publications/payroll/t4127-payroll-deductions-formulas/t4127-jan/t4127-jan-payroll-deductions-formulas-computer-programs.html