feat: add currency formatting support (issue #133)

[BohdanStarunskyi]

Add comprehensive currency formatting support

Added feature for #133

Currency Support

• 50+ currencies including all major world currencies
• Regional coverage: Asia, Middle East, Africa, Latin America, Eastern Europe
• Cryptocurrency support: Bitcoin (BTC), Ethereum (ETH)
• Precious metals: Gold (XAU), Silver (XAG)
• Case-insensitive currency code handling

Number Formatting

• Indian numbering system: 12,34,567 format support
• Western numbering system: 1,234,567 format support
• No grouping option for specialized use cases
• Currency-specific decimals:
  • JPY: 0 decimal places
  • BTC: 8 decimal places
  • Standard currencies: 2 decimal places

Type Support

• All Go numeric types: int8, int16, int32, uint, uint64, etc.
• Big number support: *big.Float, *big.Int
• String number parsing for very large values
• Enhanced error handling for invalid inputs

Usage Examples

Basic Usage

Currency(1234567.89, "USD")  // "$ 1,234,567.89"
Currency(1000000, "JPY")     // "¥ 1,000,000" (no decimals)
Currency(-500000, "EUR")     // "€ -500,000.00"

Advanced Features

CurrencyWithName(25000, "CAD")  // "C$ 25,000.00 (Canadian Dollar)"
IsSupported("USD")              // true
SupportedCurrencies()           // ["USD", "EUR", "GBP", ...]