Added new algorithm/problem in Array-DS

[rnema19]

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Describe your change:

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

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 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
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

I have added a new algorithm/problem which is quite common and asked in interviews.
Single element problem, as people find it unique to implement in Javascript this problem needs to practiced.
It's easier to implement this problem in other languages. I have added a basic solution, optimal solutions are given in the problem description.
All tests were successful.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.93%. Comparing base (08d8c6b) to head (dfa3896).

Files with missing lines	Patch %	Lines
Data-Structures/Array/SingleElement.js	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1846      +/-   ##
==========================================
+ Coverage   85.91%   85.93%   +0.01%     
==========================================
  Files         379      380       +1     
  Lines       19778    19803      +25     
  Branches     3015     3021       +6     
==========================================
+ Hits        16993    17017      +24     
- Misses       2785     2786       +1     

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