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).

Start by reviewing the linked VS Code Grammar Injection and Embedded Languages documentation, then compare the IntelliJ Language Injection approach described in the issue. Done means values for the GitHub Actions `runs` key display the appropriate embedded-language syntax when editing workflows.

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