Add diameter of a binary tree

[RinkiJhunjhunwala]

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Describe your change:

• [y ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

Checklist:

• [y ] I have read CONTRIBUTING.md.
• [y ] This pull request is all my own work -- I have not plagiarized.
• [y ] I know that pull requests will not be merged if they fail the automated tests.
• [y ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [y ] All new JavaScript files are placed inside an existing directory.
• [y ] All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames. Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• [y ] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• [ y] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

PLEASE MERGE THIS UNDER HACKTOBERFEST 2025

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 85.93%. Comparing base (08d8c6b) to head (11d6f99).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1824      +/-   ##
==========================================
+ Coverage   85.91%   85.93%   +0.01%     
==========================================
  Files         379      380       +1     
  Lines       19778    19805      +27     
  Branches     3015     3019       +4     
==========================================
+ Hits        16993    17020      +27     
  Misses       2785     2785              

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