Add Github Actions

[papposilene]

Add some Github Actions :

• Dependabot : once a month, the robot will check the project's dependencies and propose a request for each vulnerability it has fixed.
• CodeQL Analysis : at each commit (or push), an analysis of the code will allow to check its structure and validation.

[papposilene]

• Fix the path of dependabot.yml.
• Update .gitignore (add .DS_Store and yarn.lock).