Added Kahns Algorithm

[4bhayG]

Added Kahns Algortithm in Graphs Section as it was missing and it is a very essential Algorithm for Checking Cycles in Directed Graph and finding the Topological Ordering.

Fixes #1795

Checklist:

• [Yes] I have read CONTRIBUTING.md.
• [Yes] This pull request is all my own work -- I have not plagiarized.
• [Yes ] I know that pull requests will not be merged if they fail the automated tests.
• [ Yes] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ Yes] All new JavaScript files are placed inside an existing directory.
• [ Yes] 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
• [Yes ] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• [Yes ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 85.96%. Comparing base (08d8c6b) to head (a5421b3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1805      +/-   ##
==========================================
+ Coverage   85.91%   85.96%   +0.04%     
==========================================
  Files         379      380       +1     
  Lines       19778    19843      +65     
  Branches     3015     3025      +10     
==========================================
+ Hits        16993    17058      +65     
  Misses       2785     2785              

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