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).