Add locale-aware date formatter

[TrySound]

Summary

• add createDateFormatter as a dependency-free rifm/date entry point
• derive numeric date field order and separators from Intl.DateTimeFormat.formatToParts()
• support locale fallback lists and year: "short" | "long" (long by default)
• preserve partial editable input, enforce fixed two-digit day/month fields, and limit the year to two or four digits
• use the Gregorian calendar and Latin digits for predictable digit-based caret tracking
• support both insertion-style formatting and RIFM's existing replacement-style editing via mask: true
• document usage and the deliberate absence of calendar-date validation

Examples

createDateFormatter({ locales: "en-US", year: "long" }).format("12082026");
// "12/08/2026"

createDateFormatter({ locales: "de-DE", year: "long" }).format("12082026");
// "12.08.2026"

createDateFormatter({ locales: "en-GB", year: "short" }).format("120826");
// "12/08/26"

The returned { format, accept } object can be spread into RIFM options. Adding mask: true enables replacement-style editing of occupied date positions; leaving it out keeps normal insertion behavior.

Size

Measured from the generated dist/date.js entry with Rolldown minification and gzip level 9:

• 705 bytes minified
• 480 bytes minified + gzip

For comparison, the number formatter measured 780 bytes minified and 489 bytes minified + gzip with the same process.

Testing

• pnpm test:ts
• pnpm test:unit
• pnpm test:browser
• pnpm build
• pnpm exec oxfmt --check README.md package.json vitest.config.mts src/date.ts tests/date-formatter.test.ts tests/date-formatter.browser.test.ts

Unit coverage includes locale ordering/separators, partial and excessive input, idempotence, short years, and accepted characters. Browser coverage verifies replacement-style date mask editing and caret behavior.