experimental/currency: Drive monetary grouping entirely by currencyFormats data, making CurrencyEssentialsV1 a numbering-system-attributed marker

[younies]

Changelog

Current CurrencyFormatter shared the un-attributed DecimalSymbolsV1 data payload with standard decimal formatting. This forced monetary values to use standard decimal grouping sizes rather than the explicit monetary grouping pattern defined in currencyFormats ("¤#,##0.00").

This PR completely decouples currency formatters from standard decimal format dependencies and transforms CurrencyEssentialsV1 into a pristine numbering-system-attributed marker:

1. Decimal Formatter Modularity (icu_decimal)

• Added DecimalFormatter::try_new_from_payloads constructor allowing external components to instantiate internal decimal formatters directly from pre-resolved data payloads.
• Updated DecimalSubPattern::try_from_items to correctly accept affix symbol items (Currency, Percent, PlusSign, MinusSign) inside prefixes and suffixes.

2. Attributed Provider Data (CurrencyEssentialsV1)

• Added attributes_domain = "numbering_system" to CurrencyEssentialsV1 data marker.
• Embedded pub decimal_symbols: DecimalSymbols<'data> directly inside CurrencyEssentials.
• True Numsys Extractions: Updated datagen (extract_currency_essentials) to dynamically grab symbols and patterns for the exact requested numbering system attribute (e.g., arab for ar-EG) rather than hardcoding latn.
• Extracted monetary primary_grouping and secondary_grouping sizes directly from currencyFormats.standard.

3. Formatter Wiring (CurrencyFormatter)

• Completely stripped out DataProvider<icu_decimal::provider::DecimalSymbolsV1> from the trait bounds of try_new_unstable!
• Projected the monetary symbols payload out of essential data via map_project.
• Implemented standard two-step attribute fallback loading: tries user requested numsys attribute first (prefs.numbering_system), gracefully falling back to the un-attributed default entry if omitted.
• Updated compiled data constructor (try_new) to load digits directly from icu_decimal::provider::Baked.