Syntax highlight for multiline string filled with code

Is your enhancement related to a problem? Please describe.

It's pretty common to use a YAML file having a multiline string filled with code.

For instance, when is working with GitHub Actions, the run field can be filled with a shell script code:

jobs:
  example:
    runs-on: ubuntu-20.04
    steps:
    - name: Blah
      run: |
        echo "foo"
        echo "bar"

But since there is no way to use this extension and have syntax highlight on this part, it became monochrome.
It's okayish for simple code, but it makes harder to read a more complex code:

Describe the solution you would like

We can read the first line for a multiline string and check if there is a shebang.
If there is one, it imports the syntax highlight for the given language.

jobs:
  example:
    runs-on: ubuntu-20.04
    steps:
    - name: Blah
      run: |
        #!/bin/sh
        # from now on, this multiline string is syntax highlighted with the shell script grammar
        echo "foo"
        echo "bar"

No files or tests are named in the issue. Start by locating the YAML multiline-scalar handling and the syntax-highlighting integration or grammar tests, then trace how the first line of a scalar is exposed to highlighting. Done means a shebang-selected language highlights the scalar contents without changing ordinary multiline strings, with coverage for the shell example.