feat: add NepalTimezoneDate class and integrate with NepaliDate

[sajin-shrestha]

Description

• add NepalTimezoneDate class and integrate with NepaliDate
• updated README to document the new NepalTimezoneDate class and its integration with NepaliDate.

Related Issue

Fixes #104

Type of Change

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• Bug fix
• New feature
• Enhancement
• Documentation update
• Refactor
• Other (please specify)

Checklist

• I have added/updated the necessary documentation on README.md.
• I have added appropriate test cases (if applicable) to ensure the changes are functioning correctly.
• My pull request has a clear title and description.

Additional Notes

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