adding truncate_string.dart

[baimamboukar]

A dart extension used to truncate long string in to a desired length.

Welcome to Dart community

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 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}.