Add support for embedding languages for the `runs` keyword

[dekimsey]

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. 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 it seems the preferred way to do this is with an extension using Grammar Injection or Embedded Languages.

[ronjouch]

Complementing OP's request: it would be great to support several "languages embedded in YAML":

• Shell script in a run: block
• JavaScript in a uses: actions/github-script@v7 block

And "Support" means:

• Syntax highlighting
• Checking (e.g. via Shellcheck for Shell, and via tsserver for JavaScript)