added leetcode problem 18 solution (4Sum)

[Harleen-786]

Related Issue

[Cite any related issue(s) this pull request addresses. If none, simply state "None”]
Fix #804

Description

I have added solutions for the LeetCode 4Sum problem to the open source project. The 4Sum problem involves finding all unique quadruplets in an array that sum up to a given target. The solutions have been implemented in multiple programming languages: C++, Python, Java, and JavaScript.

Type of PR

• Bug fix
• Feature enhancement
• [X ] Documentation update
• Other (specify): _______________

Checklist

• [ X] I have performed a self-review of my code.
• [ X] I have read and followed the Contribution Guidelines.
• [ X] I have tested the changes thoroughly before submitting this pull request.
• [ X] I have provided relevant issue numbers, screenshots, and videos after making the changes.
• [ X] I have commented my code, particularly in hard-to-understand areas.

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[Harleen-786]

@ajay-dhangar please check the pr

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[deepsource-io]

Here's the code health analysis summary for commits 771d6ef..d7b0040. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
JavaScript	✅ Success		View Check ↗

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[ajay-dhangar]

Done