Added Calendar App

[shalini-bhandari]

Closes: #830

• Title: Calendar App
• Name: Shalini Bhandari
• Idenitfy yourself: gssoc-ext, hacktoberfest

Describe the add-ons or changes you've made 📃

Screenshots 📷

Type of change ☑️

What sort of change have you made:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, local variables)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist: ☑️

• My code follows the Contributing Guidelines & Code of Conduct of this project.
• This PR does not contain plagiarized content.
• I have performed a self-review of my own code.
• I have commented my code, particularly wherever it was hard to understand.
• My changes generate no new warnings.

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[shalini-bhandari]

@Avdhesh-Varshney kindly review it and let me know if any changes are to be made

[Avdhesh-Varshney]

Keep your project in Vanilla-JS-Projects/Intermediate.

[shalini-bhandari]

@Avdhesh-Varshney kindly review it again.
Regards