Updated depth first search iterative.js

[Rimkomatic]

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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.
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• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[Rimkomatic]

I have made some changes in the DepthFirstSearchIterative.js in Graph section .
I modified the code to immediately add the current node to the visited Set as soon as it's pushed onto the stack. This change reduces the number of times I check visited.has(neighbour) and improves efficiency.
It reduces the runtime of the algorithm which was 6.2ms on avg previously to 5.45 avg on my current environment.