Add disjoint set data structure

[its-kunal]

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Describe your change:

• [x ] Add a data structure.

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: #1583 .

[its-kunal]

Hi, @appgurueu, @raklaptudirm, I added a file for a data structure disjoint sets. Please review my pull request. Also, add hacktoberfest label in this pull request.

[its-kunal]

This pull request fix the issue : #1583

[appgurueu]

As I wrote in the issue:

We already have such a "union find" data structure (placed in the "search" folder), and our implementation of Kruskal's algorithm brings its own. How to refactor these may be discussed, but the feature is already there.

This PR needs to take the existing implementations into account; it should not introduce more code duplication. It also needs to have proper tests; the outcommented example code should be removed or turned into a proper JSDoc comment. The comments on the functions should all be JSDoc comments.