Added minimum window substring implementation and test file.

[Mayank29903]

• [x] Add an algorithm

Added the Minimum Window Substring algorithm in JavaScript, along with a test file.
This algorithm finds the smallest substring in a given string that contains all characters of another string.

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new JavaScript files are placed inside an existing directory.
• [x] 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
• [x] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation. Example: https://en.wikipedia.org/wiki/Minimum_window_substring
• [ ] 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 85.94%. Comparing base (08d8c6b) to head (64137f5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1821      +/-   ##
==========================================
+ Coverage   85.91%   85.94%   +0.02%     
==========================================
  Files         379      380       +1     
  Lines       19778    19820      +42     
  Branches     3015     3027      +12     
==========================================
+ Hits        16993    17035      +42     
  Misses       2785     2785              

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[Mayank29903]

All the files follow prettier code format, still "code style" checkmark is not updating.