added count_sort.dart

[AwsmAsim]

Welcome to Dart community

Describe your change: Added Count sort Algorithm

• Added counting sort algorithm in the sort directory.
• The algorithm can be implemented for array containing both positive as well as negative integers.
• Have run dart analyze and dart format .
• This is my first PR here. I apologize in advance for anything against the format.

• [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.
• [ ] 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.
• [x] 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}.

[AwsmAsim]

Heyy, LGTM, Can you add few test cases as well Yes, I will be doing it today

[AwsmAsim]

@akashgkrishnan I have added tests, please have a look.

[AwsmAsim]

Since the changes are approved, can you please merge the PR