• Add an algorithm

Added theMinimum 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:

• I have readCONTRIBUTING.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 butuserprofile.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.
  Example:https://en.wikipedia.org/wiki/Minimum_window_substring
• If this pull request resolves one or more open issues then the commit message containsFixes: #{$ISSUE_NO}