Add support for embedding languages for the `runs` keyword

**Is your feature request related to a problem? Please describe.**
Not specifically.

**Describe the solution you'd like**
I would like to see my step `runs` show the correct syntax when I'm editing GH Actions or Workflows. Thank you!

**Additional context**

IntelliJ supports this via a feature called [Language Injection](https://plugins.jetbrains.com/docs/intellij/language-injection.html). Users can either add a comment before the key such as `# language=sh` or modify their JSON Schema to provide the `"x-intellij-language-injection": "Shell Script"` property.

For VSCode, a request for generic support didn't gain much traction. Based on the discussion in [microsoft/vscode#1751](https://github.com/microsoft/vscode/issues/1751#issuecomment-919998146) it seems the preferred way to do this is with an extension using [Grammar Injection](https://code.visualstudio.com/api/language-extensions/syntax-highlight-guide?sa=X&ved=2ahUKEwjd1NjivqXyAhUoJrkGHQyJAq4Q9QF6BAgFEAI#injection-grammars) or [Embedded Languages](https://code.visualstudio.com/api/language-extensions/embedded-languages).

Commencez par consulter la documentation liée sur VS Code Grammar Injection et Embedded Languages, puis comparez l’approche IntelliJ Language Injection décrite dans l’issue. La tâche est terminée lorsque les valeurs de la clé `runs` de GitHub Actions affichent la syntaxe du langage intégré appropriée lors de la modification des workflows.

Rédigé par le modèle d'indexation à partir du texte de l'issue.