Implement syntax check for dla-needed.txt

During the LTS contributor IRC meeting on 25th May a suggestion was made to implement syntax checking for dla-needed.txt. The implementation requires:

• developing an understanding of the syntax of dla-needed.txt (e.g., format, date validation, etc)
• implementing a suitable script that can validate the format, exiting with an error status if validation fails
• placing the script alongside other pre-commit hook scripts in the security tracker repo
• documenting the setup for new contributors cloning the security tracker repo for the first time (as with the other pre-commit hooks)