algorithm: find the middle of linked-list

[10kartik]

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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

• Fixes: Find Middle Node of linked list functoin implementation not there #1095

[appgurueu]

This is not the form we expect algorithms to be in. Please:

• Add comments;
• Make this a re-usable and exported function;
• Add tests

[10kartik]

Hello @appgurueu, I hope the algorithm is now as expected:

• Reusable and exported function solution
• Added tests
• Changed comments

Please review and suggest changes, if any.
Thanks!

[appgurueu]

There's no need for a class here; this should be a single function or a method of a linked list class (if there is a suitable one).

[10kartik]

Hey @appgurueu, Adding a method in the existing LinkedList class made more sense.
Please review and suggest changes, if any.
Thanks!