[sarthakgupta1907-cell]

Describe your change:

Add implementation of the Armstrong Number (narcissistic number) algorithm in Maths/ArmstrongNumber.js.

An Armstrong number is a number equal to the sum of its digits, each raised to the power of the number of digits.

• Example: 153 = 1³ + 5³ + 3³ = 153 ✓
• Example: 9474 = 9⁴ + 4⁴ + 7⁴ + 4⁴ = 9474 ✓

Checklist:

• I have read CONTRIBUTING. md.
• This pull request is all my own work—I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames are in the format MyAlgorithmName.js -- PascalCase.
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• All new algorithms have tests, and the tests pass.
• All code is formatted with npx standard --fix .

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.89%. Comparing base (5c39e87) to head (4a9fb72).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
- Coverage   85.91%   85.89%   -0.03%     
==========================================
  Files         379      379              
  Lines       19778    19800      +22     
  Branches     3016     3016              
==========================================
+ Hits        16993    17007      +14     
- Misses       2785     2793       +8     

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