Add comments in Stack.js file

[Mamolets]

PR Checklist:

• [x] My submission is formatted according to the guidelines in the contributing guide
• [x] My addition is on refer on the language README.md file
• [x] My addition does not have a spelling problem
• [x] My submission has a proper and user-friendly description of the algorithm
• [x] My submission has the time complexity of the algorithm
• [x] My submission has sample input-output of the program (NOT FOR PYTHON)

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] New algorithm
• [ ] Optimization in previous algorithms
• [ ] Code style update
• [ ] Refactor
• [ ] Documentation
• [x] Other, please describe:

Briefly describe the changes in this PR

Other information:

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