Implement CBOR date tags according to RFC 8943

[Copilot]

This PR adds comprehensive support for CBOR date tags as specified in RFC 8943 "Tags for Date". JavaScript Date objects are now automatically encoded and decoded using CBOR date tags, providing seamless date handling in CBOR serialization.

Features Added

Automatic Date Encoding/Decoding:

• JavaScript Date objects are automatically encoded using tag 100 (days since epoch) for maximum compactness
• Both tag 100 (integer days) and tag 1004 (RFC 3339 strings) are supported for decoding
• Dates are seamlessly converted back to JavaScript Date objects during decoding

RFC 8943 Compliance:

• Tag 100: Number of days since the epoch date 1970-01-01 (integer format)
• Tag 1004: RFC 3339 full-date string format (YYYY-MM-DD)
• Only date components are preserved (time is ignored), consistent with the specification

Robust Error Handling:

• Invalid dates gracefully fall back to JsonPackExtension objects
• Malformed date strings or non-numeric values are handled without throwing errors

Usage Examples

Basic Usage:

import {encode, decode} from '@jsonjoy.com/json-pack/lib/cbor';

const data = {
  event: 'Birthday',
  date: new Date(1940, 9, 9), // October 9, 1940
  participants: ['John', 'Paul']
};

const encoded = encode(data);
const decoded = decode(encoded);

console.log(decoded.date instanceof Date); // true
console.log(decoded.date.toDateString()); // "Wed Oct 09 1940"

Manual Tag Usage:

import {JsonPackExtension} from '@jsonjoy.com/json-pack';

// Using tag 1004 for RFC 3339 string format
const dateExt = new JsonPackExtension(1004, '1940-10-09');
const encoded = encode(dateExt);
const decoded = decode(encoded); // Returns Date object

Utility Functions:

import {
  dateToDaysSinceEpoch,
  daysSinceEpochToDate,
  dateToRfc3339String,
  rfc3339StringToDate
} from '@jsonjoy.com/json-pack/lib/cbor';

const days = dateToDaysSinceEpoch(new Date(1940, 9, 9)); // -10676
const date = daysSinceEpochToDate(-10676); // October 9, 1940

Implementation Details

• Zero Breaking Changes: All existing CBOR functionality remains unchanged
• Minimal Overhead: Date encoding uses compact tag 100 by default
• Complete Test Coverage: 43 tests covering edge cases, RFC examples, and round-trip scenarios
• Standards Compliant: Follows RFC 8943 specification exactly, including the John Lennon birth/death date examples

The implementation handles complex scenarios including dates in nested objects, arrays of dates, leap years, historical dates (before 1970), and future dates, ensuring robust date serialization across all use cases.

Fixes #28.

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