feat: add now/today helpers with datetime conversion and time token support

[anuzpandey]

This PR adds current-date helpers and datetime-aware calendar conversion.

• Adds LaravelNepaliDate::today($calendar) and LaravelNepaliDate::now($calendar)
• Adds toNepaliDateTime() and toEnglishDateTime() with time preservation
• Extends parsing to support date+time inputs across supported input types
• Adds time format tokens: H, G, h, g, i, s, a, A
• Includes tests for now/today behavior, time preservation, and parsing
• Updates README with docs for flexible inputs, strict validation, and datetime usage