Add static calendar() method to jdatetime.date for displaying monthly calendar

[Peyman20122]

Hello dear engineer

Summary

This PR adds a new static method calendar() to the jdatetime.date class. It displays a monthly calendar (week-based table) for any given Shamsi (Jalali) year and month, similar to the Unix cal command or Python's calendar module.

Motivation

Currently, jdatetime provides date conversion and formatting but lacks a built‑in way to visualize a whole month as a weekly grid. This feature is useful for many Persian applications (CLI tools, scheduling widgets, etc.) and reduces boilerplate code for users.

Changes

• Added @staticmethod calendar(year=None, month=None, locale=None) to jdatetime.date.
• If year and month are omitted, it shows the current month.
• The method respects the thread‑local locale (Persian/English) for month names and weekday headers.
• Returns a multi‑line string (not printed directly) for flexibility.
• Handles leap years correctly (Esfand can have 30 days).
• Input validation (year range, month range) similar to the constructor.

Example usage

import jdatetime

# Current month calendar
print(jdatetime.date.calendar())

# Specific month (English)
print(jdatetime.date.calendar(1405, 2, locale='en_US'))

# Persian calendar
jdatetime.set_locale('fa_IR')
print(jdatetime.date.calendar(1405, 2))

Output example (English)

   Ordibehesht 1405
Sa Su Mo Tu We Th Fr
    1  2  3  4  5  6
 7  8  9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31

Additional notes

· The weekday headers assume Saturday = 0 (week starts on Saturday), consistent with jdatetime.date.weekday().
· The method does not depend on external libraries (e.g., numpy, hijridate).
· I have tested the method with various inputs and locales.