Add ISO8601 parser implementation

ISO8601 / EDTF Parser Implementation

Introduces a parser and validator for ISO 8601 and EDTF date/time formats.

• Core ISO 8601 parsers:
  • CalendarDate (YYYY, YYYY-MM-DD, YYYYMMDD)
  • OrdinalDate (YYYY-DDD, YYYYDDD)
  • WeekDate (YYYY-Www[-D], YYYYWww[D])
  • Time + time zones (extended, basic, optional T prefix)
  • DateTime (full date + time, day precision required)
  • Duration (PnYnMnDTnHnMnS, PnW)
  • Interval (start/end, start/duration, duration/end, incl. open/unknown bounds)
  • RepeatingInterval (R/<interval>, Rn/<interval>)
• EDTF extensions (optional):
  • UncertainDate (? ~ % qualifiers)
  • UnspecificDate (X digits)
  • SubYearDate (codes 21–41: seasons, quarters, quadrimesters, semesters)
  • DateSet ([ ... ] = one-of, { ... } = inclusive, supports ranges ..)
  • LegacyDuration (P<date>T<time>)

How to:

  >>> # Basic usage
  >>> parser = ISO8601Parser()
  >>> valid, obj = parser.validate("2023-12-25T14:30:45Z")
  >>> print(valid)  # True
  >>> print(type(obj).__name__)  # "CalendarDate"
  >>> print(obj.format)  # "YYYY-MM-DD"

  >>> if valid:
  ...     result = parser.parse("2023-12-25T14:30:45Z")
  ...     print(result["type"])        # "DateTime"
  ...     print(result["parsed"])      # {'year': 2023, 'month': 12, ...}
  
  >>> # With EDTF extensions
  >>> edtf_parser = ISO8601Parser(include_edtf=True)
  >>> edtf_parser.validate("2023?")           # True (uncertain)
  >>> edtf_parser.validate("198X")            # True (unspecified)
  >>> edtf_parser.validate("[2023,2024]")     # True (date set)
  
  >>> # With legacy support
  >>> legacy_parser = ISO8601Parser(include_legacy=True)
  >>> legacy_parser.validate("P0023-01-15T14:30:45")  # True (legacy duration)
  
  >>> # Custom parser with only specific formats
  >>> custom_parser = ISO8601Parser(include_classes=['CalendarDate', 'Time'])
  >>> custom_parser.validate("2023-12-25")    # True
  >>> custom_parser.validate("P1Y")           # False (Duration excluded)
  
  >>> # Parser excluding certain formats
  >>> filtered_parser = ISO8601Parser(exclude_classes=['Duration', 'LegacyDuration'])
  >>> filtered_parser.validate("2023-12-25")  # True
  >>> filtered_parser.validate("P1Y")         # False (Duration excluded)