feat: Added support for default formatters

[valeryq]

Add support for default formatters

This PR introduces automatic default formatter handling for ICU message format placeholders and function-based formatters that don't specify a format.

Changes

Automatic default format injection: Messages with placeholders like {value, number} are automatically transformed to {value, number, default} before parsing.
Function formatter support: The useFormatter hook and createFormatter function now apply default formats when no specific format is provided.
Generic type support: Works with any ICU message format or function-based fromat type (number, date, time, plural, etc.)
Unified configuration: Both ICU types and function formatters can define default formats in the IntlProvider's formats prop

// Before: Required explicit format specification
<IntlProvider
  formats={{
    number: {
      default: { useGrouping: false },        // Applied to both ICU and functions
      currency: { style: 'currency', currency: 'USD' }
    },
    dateTime: {
      default: { year: 'numeric' },          // Applied to both ICU and functions
      full: { dateStyle: 'full' }
    }
  }}
>
  {/* ICU message formatting with defaults */}
  {t('price: {value, number}', {value: 10000})}  {/* → "price: 10000" */}
  
  {/* Function formatting with defaults */}
  {format.number(10000)}                         {/* → "10000" */}
  {format.dateTime(new Date())}                  {/* → "2024" */}
</IntlProvider>

Implementation

The transformation is handled by a regex pattern /\{([^,}]+),\s*(\w+)\s*}/g that:

• Matches ICU placeholders with exactly 2 parts (variable, type)
• Transforms them to 3 parts (variable, type, default)
• Preserves existing 3-part placeholders unchanged
• Function formatter defaults: Both useFormatter and createFormatter check for default format configurations when no explicit format is provided

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Project	Deployment	Preview	Comments	Updated (UTC)
next-intl-example-app-router	Ready	Preview	Comment	Sep 11, 2025 9:10am
next-intl-example-app-router-without-i18n-routing	Ready	Preview	Comment	Sep 11, 2025 9:10am

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[valeryq]

The implementation doesn't fit well with the roadmap of next-intl.