Two Sum Algorithm problem

[akashgk]

Checks if there are two numbers in the array whose sum is equal to given number

Welcome to Dart community

Describe your change:

• [x] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Dart files are placed inside an existing directory.
• [ ] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[akashgk]

@StepfenShawn Please review my PR. I have been a previous contributor my handle is @akashgkrishnan. This is a my current account which i use for contributions. I do happen to have quite some contributions to the repo.

PR from previous Account. https://github.com/TheAlgorithms/Dart/pulls?q=is%3Apr+is%3Aclosed+author%3Aakashgkrishnan

Adding one approval from the previous ID