Add weekday helpers, fluent methods, and format constants

[dfang]

This PR introduces several improvements and documentation updates:

• Add weekday helper functions (Tuesday-Saturday) and refactor weekday logic via weekdayHelper
• Add fluent methods: Add, AddDays, AddWeeks, AddMonths, AddYears
• Add format convenience methods and common date/time format constants
• Add time checking and calculation helpers (IsYesterday/IsTomorrow/IsSameWeek, etc.)
• Fix README ISO8601 example and expand docs for new features
• Align formatter methods with shared layout constants

All tests pass: `go test ./...` Notes:

• MonthsUntil/MonthsSince are approximate (year*12 + month), by design.
• Consider adding tests for WeeksUntil/WeeksSince and month boundary cases.