feat: Add timezone support and extended date range utilities

• Added new functions for previous/next month and week ranges:

  • getPreviousMonthRange()
  • getNextMonthRange()
  • getPreviousWeekRange()
  • getNextWeekRange()

• Implemented timezone support with:

  • toTimezone(date, timeZone)
  • getCurrentMonthRangeInTimezone(timeZone)
  • getCurrentWeekRangeInTimezone(timeZone)

• Improved date normalization and validation

• Updated README with new features and examples

• Bumped package version