Implement Currency Formatter API (New Design)

[younies]

Changelog

This PR implements the self-contained generic CurrencyFormatter API (Option 1) as part of the experimental currency formatting architecture.

Core Architecture & Typestate

• ValueRepresentation Trait: Introduces a core trait to govern representation-specific internal data states.
• Marker Structs: Implements Decimal, Compact, and Scientific (placeholder) marker types to enforce compile-time typestates.
• Unified Formatter: Defines CurrencyFormatter<T: ValueRepresentation> which transparently adapts its internal storage based on the active typestate.

Data Models & Payloads

• DecimalCurrencyData: Added internal enum to handle both Standard (CurrencyEssentialsV1) and Long (CurrencyExtendedDataV1 + CurrencyPatternsDataV1) data requirements.
• CompactCurrencyData: Added internal enum integrating currency data with icu_decimal compact payloads (DecimalCompactShortV1 and DecimalCompactLongV1) and plural rules.

Preferences & Configuration

• New Preference Types: Defines CurrencyFormatterPreferences and CompactCurrencyFormatterPreferences supporting custom numbering systems.
• Macro Integrations: Implements prefs_convert! rules for zero-friction conversion into underlying DecimalFormatterPreferences, CompactDecimalFormatterPreferences, and PluralRulesPreferences.

Polymorphic Constructors

• Standard Decimal: Added try_new_short, try_new_narrow, and try_new_long implementations on CurrencyFormatter<Decimal>.
• Compact Formats: Added try_new_short, try_new_narrow, and try_new_long implementations on CurrencyFormatter<Compact>.
• Marker Attributes: Handles dynamic derivation of DataMarkerAttributes from CurrencyCode to properly resolve locale-specific long display names.

Formatting Execution

• format_fixed_decimal: Implements robust formatting logic returning impl Writeable + Display.
• Pattern Interpolation: Handles multi-placeholder pattern resolution, seamlessly combining currency symbols/names with formatted numbers and compact significands.
• Sign & Plurality: Correctly preserves value signs (FixedDecimal::sign) and evaluates PluralRules across all complex compact and long variants.

Testing & Verification

• Added a full suite of unit tests validating all 6 primary configurations (Decimal/Compact x Short/Narrow/Long).
• Verified expected textual outputs including exact symbol placement, rounding, compact suffixing, and pluralized long names (e.g., $12,345.67, 12,345.67 US dollars, $12K, 12 thousand US dollars).