Support setting a post hook

Start by tracing the existing github-script action entry point and how it handles the script input, then compare the node20 post-hook model and core.saveState behavior described in the issue. Done means a new post input runs cleanup after the job while preserving existing usage and supporting shared state.

Is your feature request related to a problem? Please describe.
With actions/github script it's not possible to setup some cleanup logic to run at the end of the job.

Describe the solution you'd like
A post hook is supported by node20 actions. Adding support for this would not be too difficult on github-script, as it's just a separate function that can be set on an additional post input.

The change would be fully backwards compatible and not too disruptive.

Usage would look like this:

    - name: My github script
      uses: actions/github-script@v7
      with:
        script: |
          console.log("Running my github script");
        post: |
          console.log("Running cleanup logic for my github script");

State sharing between both scripts would be supported out of the box by core.saveState, so this would be a very powerful addition.

I'm willing to work on a PR for this feature in the next few days.

Describe alternatives you've considered

• Using an additional action with if: always() for cleanup, this requires putting the cleanup step at the end of the job, potentially far from the step it's cleaning after.
• Using a node action directly: this would require duplicating parts of this project for toolkit set up.

Additional context

Use case where the need for this has come up: creating a "check" (github.rest.checks.create) at the beginning of a job, and making sure the check is marked as completed at the end (either with success or failure based on the job status)