feat(currency): Add accounting formatting and relax constructor validation

[younies]

This PR addresses what was left in #8066 and adds support for accounting currency formatting.

Changelog

1. Relaxed Formatter Validation: Removed the strict validation in CurrencyFormatter and CompactCurrencyFormatter constructors that required the standard pattern to be present. This allows the formatters to initialize and rely on runtime fallback.
2. Accounting Formatting:
   • Added Accounting variant to CurrencyUsage enum.
   • Updated CurrencyEssentials::name_and_pattern to retrieve accounting patterns (accounting_positive, accounting_negative, etc.) when CurrencyUsage::Accounting is selected.
   • Updated formatters to pass self.options.usage to name_and_pattern.
3. GIGO Fallback:
   • Implemented GIGO fallback in standard_pattern to return FALLBACK_PATTERN ("{0} {1}") if the pattern is missing (due to corrupt data), with a debug_assert! in debug builds.
   • Updated positive pattern getters to return non-optional &DoublePlaceholderPattern.
4. Datagen:
   • Updated datagen fallback for accounting_alpha_next_to_number_positive to standard_alpha_next_to_number.
   • Regenerated the baked data.
5. Tests:
   • Added tests for accounting formatting in format.rs.
   • Added a test for FALLBACK_PATTERN in essentials.rs.

Diff Base: currency-plural-patterns (which is stacked on currency-cash-formatting #8175)

TAG=agy
CONV=f08dbbb0-24e0-4da6-a2f8-f6a19ba2c8b5