feat: integrate yaml extension with .yml and .yaml files

@viktomas I did some changes in the extensions.ts file according to your instructions but it is not done yet. I am not familiar with TypeScript. Therefore I may have some logical mistakes in my changes. I am trying to figure them out during the coding process. I would like to point out some issues that I am struggling with:

1. I couldn't figure out to store the user's decision in a globalState. It is not working properly.
2. vscode.window.showInformationMessage appears only when the workspace starts with a *.gitlab-ci.yml file from the last session.
3. How this extension to be structured? How many files should I create to separate this functionality? If you are available I would like to get some help with these problems.