[Breaking] Implement Scottish Taxes, refactor public API

[sgb-io]

Adds support for Scottish Taxes

• [Breaking] Changes the return shape of calculateIncomeTax (details below)
• Updates the public APIs to accept a new, optional country argument

calculateIncomeTax now returns either EnglishIncomeTax or ScottishIncomeTax:

export interface EnglishIncomeTax {
  incomeTaxType: "England/NI/Wales";
  total: number;
  breakdown: {
    basicRateTax: number;
    higherRateTax: number;
    additionalRateTax: number;
  };
}

export interface ScottishIncomeTax {
  incomeTaxType: "Scotland";
  total: number;
  breakdown: {
    starterRateTax: number;
    basicRateTax: number;
    intermediateRateTax: number;
    higherRateTax: number;
    advancedRateTax: number;
    topRateTax: number;
  };
}